Why do people get offended over their gender?

My brothers friend said that people like saying that the boys in the group are gay because it offends them. But he says that he doesn’t get why people get offended over their gender. It doesn’t make sense to me either, my crush saw my crush and said “oh I’m gay now” and I said “no you’re not” but he said “oh yes I am” and I was just like “well you’re clearly not but if that’s what you call yourself that’s your choice but I’m not going out with a gay guy” and he just said “I’m not gay I just wanted to see if it offended you or not” and I was like “well I’m not offended by that” and he was like “yeah I know you’re not but it’s scary how comfortable you are with being called a lesbian” but I was like “my friend M is a lesbian and she’s fine and I’m friends with her and I’m not offended by her gender” and he said “well it’s offensive because I’m calling you something that you’re not” and I was like “well I’m not offended by that because I’m not a lesbian” and he was like “but it makes me uncomfortable when my friends call me a female” and I was like “well I’m not offended by it so I’m just going to say that it makes no sense that you get offended over something so unoffending” and he yelled at me to go away and said that I should just show a little more respect to other humans and I went away and am now typing this post asking about why people take offense to that because I’ve seen my crush get angry at his best friend for calling him a girl and I just don’t see it as offensive.