And the first time they do it is in very warm water of the ocean (also said to make it easier on her)
And as anyone who's *tried* to have sex in watter can tell you - it's a pretty crappy lubricant.
Salt water (and *sand*) make it even *worse*!
But if I imagine he can get her pregnant, he would have to make/have/expell semen...so probably some "vampire magic" would take over durring attempted intercourse...?
'cause sex with a female vampire would be ust as bad...like fucking a cold fish - and women need blood flow just as much as men for their labia to swell and their vagina to open (more), lubricate & lengthen...otherwise it would be like sex with a corpse...(though, some people are aparently into that...)
Really, a few added lines of dialog that lovemaking makes a vampire magically "warm up" would answer the question...(hey, if sunlight can make them magically "sparkle" - or burn up, etc...)