i don't understand

i am getting to know someone, slowly. a friendship. he's dead brilliant and a lot of fun to chat with. he has an incredible mind and a blow your sox off sense of humour. i care about how he's doing, and enjoy chatting with him because he talks about all sorts of things, not just the usual boring schtuff. last night, though, things changed.
i'm not sure why.
he started off a little more aggressive sounding than usual (we were chatting about fashion and my serious lack of knowledge) and he said that the preppy look and gay are synonymous. when i asked him what gay has to do with the prep look, he said it's all about gay and that he's gay. he signed off shortly afterward.
i'm not sure why his being gay is supposed to be a big deal. we've never talked about sex, but i don't think a person's preferences should ever be the determining factor in a friendship. i think there's something that i am obviously not understanding here.