The conception of gender and sexuality is complicated depending on the culture. Not sure how true it is but apparently in ancient Rome, it's not considered gay if you are on top.

Hmmm I doubt it, the way it varies among cultures. It's not just Christianity and Islam that gets weird about it. From what I've read it's more about sex acts that are considered taboo, like a man laying with another man, and not about some modern idea of what homosexuality is. And I believe it can be argued sometimes that we don't know if these ancient laws are about a man raping a man rather than just men having sex with men. (And if there's no victim, who's reporting it? An observer or jealous spouse?)

we're talking like 12th century BCE Assyria...

https://urania.institute/en/posts/courses/mesopotamia/assyrian-laws/

The way cultures can vary so much on what is taboo, is it that surprising that people make same sex relations taboo?

Because they didn't call it that. They still killed whoever got fucked.