Heterophobia is absolutely a thing. It's a response to a life of suffering homophobia. The real situation is that people drop people who don't accept who you are.
I think it can be that, but there's also another factor:
an in-group who is all into the same stuff can get
extremely arrogant about how the stuff they like is better than "normie" stuff. I know from being one of these kinds of people. It could be you're all into punk music, so punks are the music/fashion master-race to you, all others are inferior no-nothing normies who just don't get it. It could be metalheads, it could be sci-fi fans. It could be that you're all gamers, looking down on those normies who watch reality TV and like sports. Various forms of hipster-attitude basically, but it really pervades all walks of life.
So, I have no qualms in pointing out that if you have an all-LGBT in-group then people in that in-group almost certainly are prone to the same types of cognitive effects as other groups: a huge sense that you're in the "superior" group and everyone outside that group are inferior normie untermensch. Saying that it's a just a reaction to homophobia is a cop-out. A
lot of people who are "into <X thing>" think their thing is the master-race thing and that everyone else's thing is inferior, so I can't see how the gay scene would magically be immune to that sort of thing.
dickheads and arrogant people aren't restricted by sexuality or gender. People
shouldn't be judged on gender race or sexuality, but that has to work both ways: we shouldn't judge or dismiss someone's actions based on those things but we shouldn't give them a free pass on bad behavior either.