There's a fascinating continuity in the "sleep with the monster not the man" moral that runs through films where women are dissatisfied inside society. It shows up in Dracula as a metaphor long before it shows up explicitly as "men outside of society are outside of normal vanilla sexual practice", and it even reaches the level of parody more than once at the hands of Mel Brooks.

I think we also see it in the othering of non white characters who take on white lovers, and manage to live through the experience (not a given), because they are also not buying into the standard white= good=vanilla=a literal straight line that Hollywood wants to believe it's selling.

In depicting the sex lives of villains and the fallen, there was a lot more leeway to hint at unconventional morality, not just non vanilla sex, but female desire, kink, homosexuality, and other things that the heroes weren't allowed to enjoy, but which the directors and writers might have practiced, or wanted to practice openly.

There's probably a fascinating study in this that intersects the femme fatale with the bad boy as monsterous other and crosses over from noir into proto horror. If there's not, perhaps I have willed it into being by thinking about it too hard before breakfast.