Is describing somebody as 'the gay man' offensive?

Today, I was sitting at a table with two friends i've known for many years. One of my friends, it's widely known her brother is gay. Nothing wrong with that right? Only problem being she has 4 brothers, and one sister. I've never met any of her family but she speaks of them quite regularly. She was telling one of her stories, when I got kind of confused as to which brother she was speaking of. I asked her very politely 'which brother is this, the one who helps run a football club or the gay one?' Well, I was met with an extremely icy response. She practically screamed in my face 'How dare you describe my brother as being gay. Don't label him. Don't you dare do it again.' She acted as if I had murdered her cat. If she had one that is, i'm not sure. I found it extremely rude. Let's just confirm the situation, I have never met any of her brothers and that he is gay is the only thing I know about him. If he had brown hair, and was tall, I would have used that as a description. But as I said, i'm not sure of what he looks like exactly. Why would anyone consider me saying 'the gay one' as being offensive? Unless she is ashamed of it in someway, which I very much doubt as she mentions it often enough herself. He's gay. Yes? Is there anything wrong with being gay? No. It shouldn't even be considered a deal. It's just what you are. Yes, thank you very much, goodbye. You know? That kind of thing. It's the same as using another description like, the one who usually wears a red jumper. And what? You know. She also said that people don't use the term 'the heterosexual one' Well, look.. the majority of people aren't gay. There are lots of 'heterosexual ones' so it wouldn't be a very good description. But if I was at a place where the majority of people were gay eg- a gay bar, and a guy we all knew to be straight walked in, then yes i'd probably describe him as 'the straight one' if I wasn't sure of his name that is. Or if all her brothers were gay, and only one was straight, I would have also described him as 'the heterosexual one' I am utterly shocked at her behavior, and feel totally offended by a remark which was completely innocent. To shout and scream in someones face, is ridiculous and upsetting. I don't have a problem with gay people, being gay, blahdy blahdy blah. Whatever. It seems to me as if she is the one with a problem being offended by a term which if you had no problem with gays whatsoever would not even make you blink. This is 2011, not the 1940's. Hello?! Is what I said really offensive?