1:Vampires are dead. You are having sex with a corpse. This is illegal in most countries.
2: Vampires have super-strength. If you have sex with one, they will probably shatter your pelvis (among other things)
3: Vampires can’t get you pregnant.
4: Vampires have no blood circulation, so they won’t be able to… perform… certain tasks.
5: Vampires have no body heat. Having sex with one would be freezing.
6: Vampires will never age. You will get older and older, until they get bored of you and dump you.
7: Vampires aren’t real.
8: Vampires sleep in coffins. Coffins get cramped if more than one person occupies them at a time.
9: Vampires are pale. They probably glow in the dark. Ew!
10: Vampires are f***ing EVIL.