1) Um . . . they're dead. Sex with the dead = necrophilia. Yuck. Ok, so they have some semblance of life. But still, I just can't get past the dead thing.
2) They're soulless. You may be able to cheat the Angel of Death, but I don't think you're going to be able to cheat God. And when He says that your time's up, that soul is outta there. So what do you get, instead? Maybe a demon? Who knows? And whatever it is, who would trust it?
Yeah, maybe you can say in YOUR vampire mythology, the soul behaves in other ways. That's where I run into the disbelief suspension problem, mostly because of my own beliefs.
3) The whole bloodsucking thing. Anyone whose ever bit their tongue knows that blood tastes terrible. How gross would it be to kiss someone who has just downed entire gulps of blood? Blood has an aftertaste. Ick. They'd have to swill gallons of mouthwash, and even then, the stomach would send up fumes for hours afterward. Kind of like onions.
4) No sunshine. You'd have to rearrange your sleeping life to accommodate your lover, or you'd have to watch him play dead for hours on end. You'd never get to see what your sweetie's eyes look like in the sunshine. Bummer.
5) No sleepy snuggling. While you are sleeping, he's actually dead. Who wants to snuggle up to a dead guy?
6) The pallor. No sunshine = unhealthy pallor. And since when is a pallor attractive?
7) The fangs. Any teeth sharp enough to puncture skin is going to make kissing a hazard. Things are already bad enough with the coppery blood aftertaste, but now you have to dodge fangs as well. And if he happens to nick you, he's going to then want to feed on you? Besides, if his teeth are always hanging out of his mouth, he's going to drool. Now he's got all the sex appeal of a Doberman Pincher.
8) The perpetual youth. His eternal youth might be nice at first, but what about when you turn 40? 50? My taste in men has evolved over the years, and nowadays, I actually appreciate a bit of gray. Besides, a 50-year-old woman is going to look like a cradle robber if she's with a guy that's perpetually 25.
9) No children. Even if a vampire could father a child (I don't want to even think about a female vampire mothering a child), why would you want your child's father to be someone who is dead all day? You'd have to keep the kid up all night to accommodate the half-dead lifestyle. And children need sunshine. And they need to be around other children.
10) No warmth. I just can't imagine a vampire as a warm and loving kind of guy. Both literally and emotionally. He wouldn't have any body heat unless he just fed, and then you'd have the whole blood-breath problem, covered above. Besides, without a soul, how would he feel love? The best he's going to do is feel remembered love, probably for some long-dead girl.
I know lots of people enjoy vampire fiction, so feel free to argue in the comments. If you find vampires sexy, I'd love to know why.