There's two layers to it. One is fairly utilitarian: if you're trying to hook up for gay sex it's kind of useful to know who's going to be doing what.
The other is that i think a lot of queers don't actually unlearn the harm of sexual binarism and basically recreate the old structures they hated but with fancy fun gay colors instead.
It's fully reasonable to realize your queer identity and breath a huge sigh of relief, to go "wow, I'm so glad I've found a space where I'm not forced to be in that role." But the trap is in taking that relief and moving on without interrogating it.
Why did that suck? Because it hurt you. Okay, true. Why did it hurt you?
A common mistake i see many queers make is that they, feeling an intense resentment to their former enforced identity, respond with "because the identity was bad!"
No. The enforcement was bad. Maybe the enforcement was so bad you can never even approach similar identities again! But it was the enforcement that caused harm.
Gay men who leave enforced heterosexuality may very well turn around and say "heterosexuality was the problem!" And then recreate the most comically misogynistic "women are submissive, men are dominant" dynamics imaginable except with "top" and "bottom" replacing "man" and "woman". Ctrl+F. Because the freedom of homosexuality became about escaping heterosexuality to this strawman, not about escaping the rigid enforcement of heterosexuality.

