This made me think of all the times I came across some Buffy fusion AU in which the male protagonist is/becomes a Slayer and I'm like, no, a world of no. To me a Slayer is by definition female. It's one of the quintessential characteristic of the role, the whole narrative arc would be irremediably changed otherwise.

I haven't come across this in fanfic myself but that's largely why the "Billy the Vampire Slayer" storyline in the comics made me queasy. I KNEW the minute I heard about it that it that he would be gay. I'm sorry, but being "gay" as a man does not = less masculine/more feminine by default. So it's insulting (stereotyping disguised as ":diversity") and it makes NO sense in canon whatsoever.