I think there's a reason that there used to be websites devoted to tons of Angel/Spike smut... or Angel/Spike with occasionally either Buffy (and/or for some reason) Xander.
Spangel
Xangel
Spander
and various triad/harem/group arrangements.
There was that allusion to 'that one time' and a general impression I think we're meant to have that vampires aren't that sexually discriminate... I think there's a scene in Angel where it seems (by implication) that Angel, Darla, Dru and Spike are all getting it on together. Dru and Angel were definitely having sex in Sunnydale, and doing it in Spike's face when he was in a wheelchair...
Take home message? Vampires are kind of slutty.
It makes sense. If you're at the top of the food chain and you're a pariah in normal society AND if violence is routinely used to control those lower in the ranks, why wouldn't the various vampires practice both consensual and non-consensual sexuality? Why would it be heteronormative?
In human society, where men have had unlimited power over lower ranking men, those men were (and still are) often used for sex. In boarding schools, this behaviour may not end up in actual rape as often as in prison, but it often ends up in sexually oriented domination of the weaker by the stronger. Then, of course, there's the consensual sex, which given that most boarding institutions tend to be single gender, kind of makes same gender sex the only option.
Why would vampires be 'better' than humans?
I think when you have a power structure like a vampire court, those with power take what they want. Those without it, submit. That is what is normalized... not 'human nicety', which is a fig leaf anyway.
If you have free range vampires roaming the land, they'll take what they want (humans) and do what they want (including sex). And if they're together without supervision or society to judge, why shouldn't that include whatever permutations they choose.
Why would we expect them to follow ANY societal norms?
Even if, after one has a soul or other motivation not to practice sexual violence (or any violence other than self defense) why should a vampire confine him or herself to societal norms? Vampires are the ultimate 'want, take, have' culture.
