If you are referring to ancient Greece/Rome as a culture where homosexuality (specifically sex between men) was normalized, that's wrong. What actually was normal was men having sex with male kids, not adults (see https://en.m.wikipedia.org/wiki/Homosexuality_in_ancient_Greece). This is clearly something we now consider a complete abomination, obviously, but men did not use to have sex with other adult men (surely there were cases, but it was not a widespread cultural phenomenon like pederasty).

This is also why I don't think it makes sense to use historical references to help build a moral foundation about what's right and what's wrong. Gay sex is fine because it's consensual adults enjoying and not harming anybody, not because there was some golden age in which this was accepted.