Two words. Monster boys.
There are thousands of sexy monster ladies like harpies, slimes, lamias, etc. Always female, to the point that most works do that dumb “they are an all female species and need human males to mate” thing, because we totally need a reason to put a dude and a lady together in NSFW to begin with, right?
Instead of sexy monster boys we get big nasty fat pig men and like gargoyles and stuff. Stuff with like a male slime or a male harpy is incredibly rare.
What it boils down to, it sucks hard to be a gay man in the whole “monster people” crowd :(