Ancient Homosexuality, why did it become "mainstream" at different times in different cultures?

Still, I think there’s an argument that modern homosexuality and pederasty do not describe the same sorts of relationships. There isn’t the inequality involved, nor an idea that you should give it up as you get older. And it is treated as equivalent to heterosexuality.

It’s possible that what allows it to become “mainstream” this time isn’t the same.

That’s true, but modern heterosexuality and ancient forms of marriage also do not describe the same sorts of relationships.