Or perhaps those other cultures are simply "more homosexual". In Ancient Greece, literally every male was bisexual. I don't see any reason why we shouldn't call people who engage in homosexual activity because of culture homosexuals or bisexuals just because they were culturally influenced into doing it, and not on their own. It just means that the culture made them accept homosexual acts as something normal and then that premise made them capable of having romantic feelings for the same sex. Which Ancient Greeks certainly did have.
In Ancient Rome, as far as I'm aware, it was more of a case of them realizing gay sex is something pleasurable and then using that fact to just indulge in gay sex regardless of feelings. And we all know how much Ancient Romans were obsessed with sex and pleasure. xD