This is actually complicated. So here is my own opinion.
First I will ignore magic. Though you do have vampires.
Second sexuality is really really complex. Why do we get turned on by the things we get turned on by?
You can't explain every single detail with waving your hand and saying: BIOLOGY! or an evolutionary reasons we will find later. Obviously we can argue but the thing about intelligent life is that we truly transcend our biology in a way or in certain actions and so on. But anyway take of that what you will.
Anyway we humans have weird tastes. I'm no vampire and I find blood very erotic.
People use and used pain to enhance their sexual experiences for a long time.
All this is not exactly new. So being turned on by blood or feeding could actually be a fetish or a trait passed down from the mother/father/whatever vampire to the rest.
It could also be social in a loop of sorts. See. First vampire says it's hot. New vampires are not convinced but first vampire enforces it's hot. You grow up thinking that's hot. So it actually becomes hot.
You can see this in many cultures who may think small feet or a large nose or whatever else is very erotic. So with time and as the later generations start practicing that thing it just becomes the norm. Culture controls a lot and with vampires in a clan I suppose the control of the main vampire is higher.
My favorite answer:
Feeding is not eating, it does not activate the eating circuits in the brain nor is it even close. Feeding is hunting, and feeding is domination.
This will take a darker turn but it's very close to what vampires are, at least the classical predator and night fiend.
You see much like how sex in prison is never about sex or that rape or sexual harassment is also never about sex.
Or you can argue levels of that, but point is that in all these cases what you get is the domination of the other person and the person doing the thing just being drunk on a form of power lust.
So it's the same here. Your vampire consider, or it's wired in their biology whatever, that hunting and feeding is really about being a strong alpha predator that hunts the prey then have his/her/its will with the prey.
It's domination so they get turned on by that. You can do whatever you want with that. Have their wiring all messing up so it is equally both, or whatever ratio.
This last point could also be used for many, but not all human.
Some people prefer to be dominated and find pain quite erotic.
Perhaps your vampire are actually more naturally inclined to hunt and and feed on those people.
I mean it does fit the predator description well.
You don't want a strong prey, you want the weakest of the heard as it will offer the least amount of resistance.
The experience is akin to religious or transformative ones.
I'm still ignoring magic. But a quick look on certain religious practices you can find that pain becomes a sort of a pleasure of it's own in the context of cleansing the soul or appeasing god or whatever else.
Beating yourself or starving yourself or going through a sort of hellish torment or something similar are all things people did/do.
So maybe your vampires activate this sort of feeling in them or in their prey or both. A vampire feeding is seen as a part of their duty to the clan or god or whatever. And the humans are also, maybe, feeling like this will turn them into an eternal creature? Perhaps new vampires are those that survive or let survive from the feeding. This will mean a shared power or magic or bond or whatever connection between vampire and prey that both experience once their fang is in.
It's like telepathy only using the fangs to have this vampire/prey connection.
Not magic mind you. It's just using what is hard coded in the premise and expanding it.
Clan/magic/vampire limitation on feeding.
So you know how a vampire can only enter if invited?
Well. Maybe a vamp can only feed if invited to "have" the persons body.
So the rules, whatever their origin, require the vampire to seduce the person in order to gain access to their body. Because you can't walk up to people and tell them you want to suck their blood.
So the only way to feed was to seduce the prey and have it completely hand control of their body to you, thus removing the limitation and allowing feeding.
Overtime you obviously see how the two are mixed in together.
The vampire have to seduce the prey and so they are thinking of them sexually and the prey is thinking of having sex. Heck. Maybe feeding does not even hurt that much.
Lastly, sorry couldn't resist, MAGIC!