I would like to know who was the first oh-so-clever person who happened to wake up one morning and think to themselves... 'Oh yeah, you know what we need "Sexy Vampires" that can get all rude and dirty and stuff... and maybe don't go poof in the sun, cause that would be a dull story. Maybe we should just have them have a lot of sex and squirt their non existent Vampy love juice all over some kinky types.'
So if Vamps are - undead - cannot have babies, don't have a working circulatory system... How do they get it up? Erections need blood, it pumps from one head to the other (yes, I know it's more complicated than that). Wait for it, I can hear your cogs turning... Don't you dare answer with - "They drink blood!" - Shame on you, I eat steak, does this fuel my rampaging man-thing? Cause it doesn't (well it does but, once again it's complicated).