I think it's a matter of demographics. They over generalize to what they feel is the target sector for this.
Maybe they also freak out a little, seeing a male in a sector that is hard for a man to accept being into.
I'm just guessing, I can barely stand yaoi, but I think it's the same thing that happens when you see a girl in a nerdy environment. She doesn't belong there, and she's messing up your perception of the universe.
NO GIRLS ON THE INTERNET!
Another idea. Maybe they don't want to be confronted with the reality of homosexuals, that must be nothing like the over idealized images they are used to. Pig disgusting 3D goes all ways.