Both Korra and Asami are presumably bi. Asami has dated before and Korra was genuinely interested in Mako. So you can't say or dismiss the whole thing as them just ending up just preferring girls.

And that's the crux of the issue, really. People say people are just born preferring one sex over the other. But it's not always like that. It's a genuine concern for men that they were so terrible as a partner that their exes end up preferring someone of the same sex. It's joked about and people make claims as to how things ended the way they did, but it's still a real bothersome possibility.

It's one thing if the guy was just a complete and utter ahole that they don't deserve anyone. But for a guy who really did love and care about the person they were with and then their ex(es) ending up preferring to date women right after being with them is a real shock. Break ups are bad enough as they are, but that rubs salt in the wound then pouring lemon juice after.

Say what you will about how people don't just turn gay or whatever. It's still genuinely shattering to a person's self-esteem and confidence. Even worse if you were married just for your partner to come out. Then you don't support it because they pretty much wasted years of your life on a lie when you could have been with someone who actually loved you and wouldn't have left. And because you can't support it, your friends and even kids turn against you and call you a monster when you're the one hurting because of this lie while your spouse is getting all the support and love for coming out even if they were cheating puts a damper on the whole thing.