I owe my writing career to the fanfic of Xena:WP. Writing as PruferBlue about pre-conceived characters actually assisted me as a new writer. When readers gave very positive feedback, I developed the confidence to continue. I made a lot of mistakes, but as my writing continued, so did my knowledge.

Quite a few lesbian writers started in fanfic — it was one place where we were accepted and embraced. So don’t feel guilty. Be sure to tell the fanfic authors that you’re enjoying their stories. We all need encouragement.

The alt-hist equivalent of today? Would be v hard — if that kind of heirarchy had been in place and working for 25,000 years, you’d have a hard time breaking it now — just as in our world you’d have a hard time convincing people that dogs or horses are equivalent to humans, rather than working animals.

Stone Age? I think Jean Auel did something similar to that, only not gay.

“Hatred against people just because of their skin color or religious beliefs” is an age-old and universal problem, but it’s generally due to culture clash. Blacks in Africa living perfectly happily plus Whites in Europe living perfectly happily => slavery when White meets Black. Religious hatred is often an excuse to go to war over territory. But no oppressed or occupied people to my knowledge have spent tens of thousands of years accepting their inferiority and believing it because of their race or religion. The closest analogy I can think of is the Indian caste system; you might want to look to that for inspiration when you do your world building.