I think it's dumb, and I'm not even gay. Guys pretend like they don't know when another guy is attractive even though they do. As long as you aren't hitting on them, I don't see the problem with compliments.
It honestly depends on the way you say it, you can't say "OMG he's so hot, i'd go gay for him" and not expect to get a negative reaction (unless you are gay).
I can name so many dumb reasons, and that is NOT a good thing. Mainly tho, "masculinity", insecurity, sexism, homophobia etc.. also, what RastaMantality said.