How could demons of lust affect/tempt beings that do not reproduce sexually, and have no sexual desire? Like, they can't even be affected by lust magic, their brains simply aren't wired for it. How could the demons tempt them anyway?
You are asking for a logical absurd, I think.
If a being doesn't have sexual desire, it is straightforward that sexual lust is a non concept for it.
The only way would be for the demon to turn the being into a sexuated one, and once the hormones have done their job, tempt it with the arts in which it is skilled.
Asexual creatures might not have lust, but they have an urge to reproduce - at least real biological ones do. So your demon could trigger all the behaviours which lead up to asexual reproduction. Amoebas want to feed, grow and split in two. Greenfly want to feed and give birth as often as possible.
If your creatures are non-biological, like robots or golems, perhaps they'd be possessed by the urge to build more of themselves.
If you take the wider definition of "Lust", you may affect them with some other aspect of life. Maybe a good wine, extravagant clothing, luxury and the like. Every desire considered sinful in that society could be stuff to work from.
Maybe the demon could tempt them into "worlds of emotional pleasure unknown". That does not have to mean sexual lust to them! Maybe the things they are known to.
Posted answers so far are sensible. Another approach to this would be to have the demon magic act like a short circuit. If you give power to a circuit that does not exist or is wired in an unexpected way, you can get unexpected results. The turn signal turns the windshield wipers on. The horn comes on instead of the headlights.
In your species, the applied magical juice has unexpected effects. Certainly something is going to happen. What that will be is not predictable. That will be good for a story!
Cheers for making a question that is difficult to answer while at work!
Please exchange "Almost Aggressive base desire for another being" with a certain S word that my company's internet security will be flag up as inappropriate!
If it is specifically "basal desire for another being" lust that you are wanting, then the simplest way is to have your target species previous have been capable of regular desire toward another, and then over time that species evolved more towards A... sigh... procreation with themselves... over time the hormones that drive lust or regular desire toward another in this species stopped being produced, but the recepters are still there, then all your demon has to do is through handwavium inject some of that hormone into the creature and then they'll experience the lust you want.
Most likely they wouldn't know what to do, but oh well.
Other than that other have already answered perfectly good work arounds
Speaking from a human perspective. There are some people who claim to have no sexual desire, and call themselves asexual. Part of the LGBTQA spectrum. If they were told they didn't or couldn't have any sex at any time for the rest of their lifes, they would be content. Unlike the vast majority of the human population!
However, while asexuals don't desire any sexual relationships, some do require and desire romantic relationships. Ie, they have a mental desire for a romantic relationship with a partner of their choosing. Eg someone could be asexual and biromantic or asexual and heteroromantic etc. See the asexual forum Aven and others for more information.
Your asexual aliens could simply have a large desire for deeply committed romantic mental relationships with a partner, that has no sexual conotations. Your lust/desire demon could ramp this romantic desire up to 11 or down to 1. Causing all sorts of issues where the affected alien becomes super clingy or soppy or jealous or stalkerish etc. and the romantic partner is put off by this uncharacteristic change. "You are not the person I the fell in love with, Goodbye!" Breaks the affected alien's poor romantic heart.
Just because a being is asexual does not mean they cannot lust. First you have to define assexualism in more detail.
A good example of this is an ongoing show in Cartoon Network: Steven Universe. The main characters are assexual aliens. They do not reproduce sexually, but are rather built from raw materials. They don't even have the concept of sex. There is only one ever known case of one such alien learning about sex and doing it, and it is unknown if she enjoyed the act. Most of their society loathes organic life and things associated to it, and the aforementioned act is considered more than taboo and one of the worst criminal acts in their history, punishable with death.
As you can see, they dislike sex more than the average assexual human.
Yet they are able to perform an act which is even more ecstatic than sex: two or more aliens can temporarily fuse into one. For the time they are fused, they become one single being, with a new name and identity. This is seen as the supreme act of intimacy, and many aliens have a drive for it that can only be described as lust. The act of fusing usually involves a dance, which may be sensual or erotic for human standards.
Drawing from this exame, if a creature does not reproduce sexually, but has some sort of social interaction which can cause an ecstasis similar to what sex does to humans, the demon can capitalize on that. Some beings might have a lust for intimacy rather than for coupling, others might orgasm only to music, whereas others might be turned on by roleplaying in a non-sexual way. Even potatoes might do the trick. It then becomes just a matter of finding out what the demon's mark's kinks are.
could the demons tempt them anyway?
By showing them what passion and sex feel like, make them vision of having sex with someone they love, even if they don't love anyone...
Dreams can be quite realistic, for example more than once I had erotic dreams and I literally felt the exact same sensation of real sex, before I even had sex for the first time.
So here's an example from Dragon Age. The Desire Demon. And do you know what most people desire? How most people get trapped? Mostly, it's home, safety, family--not sex.
So this really depends on your demon. If it's simply a succubi or succubus, then they are a SPECIALIZED desire demon, and if that's all they know how to do then--sure--an asexual person would be difficult or near impossible to use/trap/seduce.
Lust is a word that can be applied to wandering, to power, to many, many other things. Desire isn't just sexual. People want a caring and loving mother, the desire demon pretends to be her, acting in just the way they've always wanted, not the reality. A heart's desire can be many things. We can lust for many things.
Even if there's no sex, there's bound to be intimate relationships. This is all dependent on how the demon's power works.
In Pokemon, various species of Pokemon can have genders, and some of the moves available use the gender differences against their opponent. For example, the move Attract will give a status condition where the afflicted Pokemon will have a percentage chance of ignoring the attack and doing nothing for a round if the opponent's Pokemon is of opposite gender, regardless of species. It's essentially a lust increasing attack.
A Pokemon species can either be gendered or genderless. For those species without gender, the move Attract (as well as any other handful of moves that are gender dependant) will simply fail if used on a Genderless Pokemon. I would recomend this. Biologically Asexual species (species that reproduce without sex, as opposed to a individual member of a sexual species that has no compulsion to engage in reproductive activity) have no mechanisms to induce a state of lust (i.e. Hormones) as these mechanisms would biologically tax the species without benefit. Evolution favors survival efficiency, and thus every part of a biological creature subject to the rules of Darwinian Evolution (as opposed to Pokemon Evolution... which isn't a thing) is a feature that is optimized for survival, fuel intake, or reproduction of the species.