Just rewatch the scene where the Master paws and bites Buffy, it's more her reaction than anything else, she's like a pornstar faking it (as indeed she is in Get It Done).
As I recall Spike doesn't actually get to bite Buffy in Out of My Mind? As for Dracula it was at least semi-consensual, the point being that this is a matter of seduction, whether we think they just had the biting or actually had sex? (what I always wonder is what happened between Dracula and Joyce?)
As for Joss, I remember watching the scene in Alien; Ressurection where Ripley get's carried away but the drone...
I think one of the reasons why sci-fi, historic and supernatural erotica is so popular is because it isn't real in the actual world of today and never can be so we can enjoy the fantasy guilt free,