It's a big pet peeve of mine, too, though thankfully I don't see it as often these days as I used to. I did see one McShep Match fic with a warning for slash, which just made me headdesk because it's so STUPID. It's a fest specifically for a slash pairing! (PS The fic was also horrible. This did not surprise me at all.)

I actually see more warnings for het these days, which enrages me just as much, because it always feels very misogynistic. Watch out! There's icky girl parts!