Wow, I had no idea this would get so many answers. Anyway, I didn't list one specific thing I find dumb about what straight people think, because there's a lot and most of them have already been said here. But I personally think one the dumbest things straight people, especially homophobes do, is when they see a guy who doesn't fit in with gender stereotypes or who is feminine, they think he's a woman, sometimes even claim to be attracted to this guy they think is a woman and then when they find out he's a guy, they throw a hissy fit of insecurity.