I have a theory that it goes all the way back to Stonewall. No, Stonewall was not the start of the Gay Rights movement, but it may have been the point where we were assimilated into the larger Counterculture movement.

The same movement that spawned identities such as beatnik, hippie, freak, punk, goth, and emo reinvented itself in the 2010s as queer. If you think about it, even the rainbow flag is more of a counterculture symbol, having been used previously as both an antiwar peace symbol and in the Civil Rights movement's Rainbow Coalition.

Once gay marriage became legalized in western countries, gay people fell out of favor with the counterculture movement. Counterculture has since moved on to trans issues.