Human buildings usually have one or more doors which combine the function of an entrance and an exit, emergency exits being the only notable example I can think of, albeit designed for contingency use.
My fictional alien civilization builds houses with separate exits and entrances; even toilets and closets are outfitted with two doors each.
Why?
The body shape of the aliens makes it difficult to pull doors open, but they can easily push doors open (or vice versa). Have you ever tried to get through a door with both hands full? The aliens' body shape has the same effect.
So doors are generally designed in pairs for both directions.
If you were to attempt the use of a door at the same time as me, one of us must either retreat for the other or turn their back to walk out of the way. One is a dishonour to me: retreat is never acceptable. The other is an insult to you: it implies such weakness as to not be a risk or danger.
While many of the raw aspects of such a culture have been refined, since constant bloodshed does tend to reduce populations, it is only stabilised by adhering to very rigid principles of honour and avoiding any instance of disrespect to others or requirements to dishonour one's self.
Couple this with
other rules based on the same principle
the existence of gods whose worship occasionally conflicts with the society rules,
new cultures who don't adhere to this obviously necessary design feature,
the usual snowball effect from one minor incident involving someone with more power than brains and the protagonist,
other pressures in the world that make following the rules more and more difficult,
and bam - tension to drive the story!
Feng Shui!
In the traditional Chinese Architecture:
The main gateway entrance into the courtyard house had two red doors; it was located on the eastern part of the south side conforming to the rules of feng shui. Immediately behind the entrance was a solid wall '照壁 zhào bì' that denies direct view or access to the courtyard, in Feng Shui terms it was to keep the evil spirits out (and cold winds too).
I can perfectly imagine a culture where going back or backward is seen as cowardly, retrograde, inciting ill-luck, ...
I can also imagine a culture in which doorways are adorned with a blessing on one side and a curse on the other. It is ill-luck to see the curse, and therefore people only enter the doorway from the blessing side.
Change
Have you considered that maybe marking the change that occurred is important:
In this case, it could be that all locations that have a purpose have a doorway representing each state. For example, imagine a shop:
Plenty of constructions have separate entry and exit sites. There are two main reasons.
1: Flow control. Freeways have separate exit and entry ramps because of the possibility vehicles exiting might interfere with vehicles entering. Sites handing large flows of people do the same - for example theme parks or concert venues. Flow is smooth because persons exiting do not have to buck the tide of those entering.
2: Functions associated specifically with entry or with exit. Airports have separate entry and exit so persons entering can be screened before accessing the interior. Large stores (for example Home Depot) have separate exit and entry so people can pay for their purchases before exit.
The latter would probably apply to your aliens. There is some function which must take place before entering or exiting. Maybe it has to do with conditioning the interior environment or the new entering occupant - for example I can imagine a "mud room" associated with an entrance where persons dirty from the outside clean off before entering; no need for this on exiting.
3: Yes I added a 3. I could imagine an alien which would not want to cross its own path for a time because its presence depleted or contaminated the local environment in its path. Examples might be something which breathed up local oxygen in the water. Or an alien, like my friend Paul, that farted a lot as it went along and did not want to double back and breathe those farts.
No ability to walk backwards, only turn with a notable arc. If they evolved feet that weren't coordinated enough to just turn around, they might need to avoid meeting someone at a door where they have no ability to back off.
Cultural reasons were mentioned. I would say that religion and superstition could definitely explain it. If they are like the Centauri of Babylon 5, they might have "gods by the bushel." So, there could be a god of entering and a god of exiting. Using the wrong door would show that you favor one god over the other, leading to bad things happening.
They could also have a really strong "don't look back" culture.
The interesting thing would be if the bias is so strong, they don't bother locking the exit door because no one would use it to break in.
Traditionally, doors are placed at the end of corridors. This is a consequence of having to defend homesteads against bands of marauding bandits. Long corridors were made in front of doors to act as chokepoints so that small families of defenders could hold off larger forces. While society has developed so that these are no longer necessary, it is considered bad luck to not have a corridor in front of every door.
As for why two doors, have you ever seen a quadruped try to turn in an enclosed space?
The OP's title calls the entrances and exits "portals". If they are more than physical doors it could be a reality of physics that only allows them to work in one direction.
Because a fee must be paid for entering into any building.
There's a system of turnstiles in Russian public transportation for this exact purpose (in Moscow and other cities). On trams and buses, only entrance through the front door is allowed (immediately followed by a turnstile everyone has to pass). On railway and subway stations, all platform entrances and exits are guarded by separate turnstiles; all of those are already existing buildings with enforced separate entry and exit.
A turnstile by itself doesn't always have to have separate doors/turnstiles. But when a fare collecting also happens, it slows people down at the turnstile, and a queue of them may form, blocking the doors. Separate exit path prevents entering people to become an obstacle for exiting people.
Now in some other place, all the buildings could be organized in the same way. It could be a tax imposed by the local tyrant. Or we can imagine a culture devoted to obtaining profit. The latter is even more likely to separate entries and exits everywhere: everyone wants to imitate big, successful palaces and corporate headquarters that have so many people entering and pausing to pay that a separate exit is required.
Real World example:
There is a religious sect here in New Zealand (and, I'm sure, in other countries too) that are very private and have some intriguing rules about tech and stuff (as religious groups tend to do). My mother's (relatively normal) church purchased one of their old sites, after they left town for some probably totally normal reason, and just started churching in it.
One interesting feature of the building was that all the bathrooms had an in door and an out door, and the reason was supposedly that even if you wash your hands, you're still unclean to an extent and both doors were designed to be pushed without needing to use your hands.
Why this design didn't apply to the other doors in the building? Who knows?
Actual answer
Perhaps your alien race just have a very old hang-up about touching things that hundreds of other people have touched and design their doors to be pushed without their hands.
The inhabitants' main form of locomotion is to float through the air using their biological gas-bladders. They are naturally inclined to moving upwards and thus live against the ceiling of their homes in much the same way legged creatures live against the floor.
They can expel gas to alter the composition of their bladders and so float down to the ground. However this carries disadvantages. First it forces them to rely on their weedy atrophied legs for walking, at least for the time being. Second it is a one-way process: They cannot re inflate in a hurry.
I imagine this is a natural defence mechanism against aerial predators -- blast them with foul smelling gas then drop out of the sky. I also imagine doing it in public is regarded as rude in much the same way we regard public defecation as rude.
The homes have an exit located near the top and an entrance located at ground level. A typical day the owner wakes up, floats out the exit and goes about their day. In the evening they de inflate and enter the lower entrance. They sleep for several hours at night, enough to generate more floaty gas, and wake up against the ceiling.
Aggressive environment.
It could be microbes or predators, but both entry and exit have different functions.
Entry
In case of predators, the entry is excellently visible (warning color, lights) and can be easily found even when confused or visibility is low, it can be opened at once and allows a panic button to close the door immediately and with great force. It also has weaponry to defend themselves and a lock construction, so even when a predator enters, it can only maul the unfortunate inhabitant and not the people inside the house.
In case of microbes, here are the sensors to detect them, a specific shower to clean them off before entering the house and some container to store the clothes which will then be rolled to the exit.
Exit
In case of predators, the exit is perfectly camouflaged and allows to safely scan the environment for their presence from hidden eyeholes.
In case of microbes, the exit allows to put on the clothers which have been thoroughly cleaned. The exit area has overpressure, so microbes cannot enter the area when leaving.
While it is still possible to build some functions in both entry and exit, experience shows that it much, much easier to hold something in perfect shape if it is specialized for its purpose. So different entry and exit makes perfect sense.
Collisions are costly and/or painful
For some reason related to their physiology, these aliens do not handle physical impacts well. Perhaps they have some sort of weak, gaseous bladder on their front, possess naturally frail skeletal structures, or have a protruding appendage that is very susceptible to pain. Since most doors are opaque, the risk of injurious collisions from beings trying to use the same door at the same time is too great, but within a room it is much easier to avoid such a collision. In situations where privacy/security is less of a concern, transparent doors could replace the two-door system, but it would also be easy to point to tradition or to the old laws that enforced the two-door system (which perhaps predate transparent materials suitable for constructing doors) to explain why they don't take that route if you want to avoid it.
This might also lead to to some changes in how they handle corners, whether that involves mirrors so you can see around them, utilizing more round structures, or having a lane system (whether physically drawn or just implemented as common sense) where you always stay to the right when approaching corners or something.
What are these buildings made out of? The first thing that came to my mind was heart valves. They are unidirectional because our blood needs to flow that way to function efficiently, but it's also really hard to imagine a bidirectional valve of that type.
So maybe these aliens live (or at least used to live during the development of civilization) in buildings composed of something resembling free-standing cardiac tissue that they are able to grow to the shape desired. Or at least some material that is tough but also flexible.
Causality
The aliens inhabit a distorted region of space-time that means they can only move in certain directions without breaking causality.
See Greg Egan's Into Darkness.
Observant Jews won't use the same plates for meat and dairy. As I understand it, separate ovens aren't unheard of (apologies if I've garbled details; most of my knowledge of the subject comes from a former colleague, not a halachic scholar, telling funny stories about the armed truce she'd reached with her observant mother in law).
So, people can do things for the sake of ritual cleanliness that might seem bizarrely arbitrary and counterintuitive to the uninitiated.
But really, doesn't any civilized person understand that you wouldn't want to eat and excrete out of the same orifice? Likewise, nobody with any self-respect or dignity would tolerate living or working in a building that used the same -- please excuse the coarse language -- door for entry and exit. It's just... obviously not clean. It's a disgusting thought. Ugh. Gross. Just... yuck.
A very simple answer: Locking mechanisms.
Doors in their society are designed to only be opened from one direction, to make invasion more difficult. However, they also need to be able to quickly flee if their lock is broken.
As such, entrances are designed to be opened from the outside only, but lockable from the inside. This also applies to spaces such as toilets, in which the occupant seeks privacy. The exit doors are then set up so that they can't be entered through - therefore, such doors are inherently secure, but will not prevent escape from a situation.
Of course, this is the original reason for the approach. Tradition maintains it for all door systems, for two reasons - you never know when you'll need the locking system, and because it also prevents two people trying to use the same door in opposite directions anyway, and thus it has simply become a standard.
Older houses would have the exits well-hidden to prevent people from easily finding them from outside, but it has become "hip" to have exits close to entrances.
Highly hierarchic society
Evolution from hives
Evolution from a flying or other high velocity specie
Aesthetics
Maybe they are genetically programmed to build geometrically symmetric colonies.
An unsymmetrical apartment is repulsive to them, and hence they build symmetric infrastructure even after developing more modern technology.
Maybe they like to have a "flow" in their motions, processes and actions. i.e. they have a specific building that they go into to pick up stuff and if they go out the same way the went in they create a choke point in the flow, the cause a disruption. Also, in the case of toilets it makes it more hygienic since they sanitize their hands afterwards so their hands are cleaner then when they went in so if they touch the entrance they negate the effect of sanitization. This "flow" allows them to be more functional and efficient.
Because that society is obsessed with logistics.
Not only do they have separate doors. It is that every aisle, sidewalk and road are of a single direction, so they do not make them wide enough so that someone can pass in the opposite direction.
Restaurant kitchens also use one-way doors, so that the doors can always be pushed, without pushing them into someone going the other way -- perhaps the aliens' hands/tentacles/etc. are always in use when they go outside (this could be for a reason as mundane as incessant rain and umbrellas, or as fanciful as you like) . Your aliens could even lack a multipurpose appendage, and rely for construction on telepathically enslaving some lesser species which isn't numerous enough to provide porters for all doors - pushing is the only way to open doors then. They may have a suitable body part but keep it private.
When the aliens were led out of bondage in Misr, their deity killed off all the first-born children of Misr. So how would a deity know which house had children to be spared, and which were to be killed? The aliens simply dabbed a bit of ruminant blood on their doorposts to inform the all-powerful that this house has children to be spared.
To this day the aliens celebrate how their children were spared by placing not blood, but a parchment, on every door. Tradition dictates that they honour this parchment each time they pass through a doorway by placing it close to their heart. And since it would be such a pain to take down the parchment, hold it to the chest, and place it back again each time they go through a doorway, instead they simply touch the parchment with the sixth finger of their left hand, which conveniently has an artery coming directly from the heart.
Since the aliens use their left hand to honor the parchment, they must always go through the door in a specific direction. The parchment must always be on their left.
Cultural reasons are all well and good, but feels like a handwave. Behavior is more believable when it has a practical, comprehensible rationale behind it.
Reasons we do it in human society:
From this, it seems that the best example we have is the vehicle. If aliens have eyes at the front, and find backing up or turning around in confined spaces to be awkward, perhaps from being long (someone suggested centaurs) or from being spined or shelled, then they are likely to build houses in a circular fashion, with a round central courtyard, then a corridor inside, then rooms arranged for suitable flow of movement around that corridor, each with an in and an out door. Their lives would be set up to facilitate this: different rooms at different times of day, perhaps following the sun.
Operations like making a coffee would be arranged as linear process flows along the countertop.
Causing someone else to have to back up or turn way would be considered rude.
Perhaps all houses are "clockwise", or perhaps different cultures are set up to flow around their houses in opposite ways. Perhaps they alternate on each level. Perhaps some are figure-8 or cloverleaf. The important thing is that people can flow easily around their property without having to turn in place, or obstructing others (thus, passageways should be wide enough for two to walk abreast).
Some buildings are required to have doors that open outwards, for evacuation purposes. The planet could have frequent natural disasters or events causing need for an evacuation.
Why would a civilization outfit buildings with separate portals for entering and leaving them?
If they are actually portals and not doors, there might be several different reasons. Maybe the portals are one-directional only. Or if they are both directional, there might be a risk that you bump into someone trying to enter from the other direction (if you can't see through the portal).
