He brought his ex up in conversation, and he overheard her talking about my friend, saying all these mean things about my friend. I just said, "we'll your ex girlfrien has no right to say those things about my friend, especially since she is downright ugly and fat." Well I thought he would agree, but he got so defensive and mad. He said "are you trying to say I only go out with ugly girls?" Why would he be so mad? If he's over her, then I would think he could agree with me. And for the record, his ex really is fat! I was stating the truth
You insulting her, is in turn insulting him, or so he feels. His ex shouldn't have opened her mouth about your friend, but you're just as bad now, by insulting his ex. So he puts up his defenses because of your statement, and sees you as less attractive, because you're calling out another person, that's a sign of insecurity and meanness at heart. Guys don't want a mean insecure girl, they want a kind hearted girl who has confidence. So you quite effectively made yourself seem less of a gem in his eyes, nice work.
I've been in the similar situation before in the past, most people try to defend their ex if they care about them in the slightest degree. Its like you have that one relative not everyone likes but they will still come to their aid if they were threatened.
I guess it's because the truth hurts. if somebody made comments about one of my exes ie. fat, stupid, idiotic, etc. I'd be hurt and totally feel like they were saying I'm a fool for even going out with someone like that or going so low as to date someone like that. and believe me it's happened as in comments made like that and it hurts. hope you two can move on without thinking or talking about his ex again. take care.
My guess is it more likely has to do with the fact that he thinks it looks bad on him. I dated a guy 3 years ago and when my friends say he was so ugly and kinda gross. I get upset and stick up for him because I think it makes me look bad that I chose him. I'm not still interested in him. It just took a long time to accept that's what I chose.