It's awkward: Here you are with most of your clothes off in bed with this person who you've really just met. You're strangers to each other's bodies and you're coming together for the first time in front of all these people.
I always thought of vampires, especially the young-adult ones, as a metaphor for sex - sucking blood, forbidden, taboo. I think they just ooze sex. Vampires are all the big themes in life in one attractive, bloodsucking package.