a: the historical records of many cultures have been scrubbed of evidence of homosexuality a handful of times in history (either through casual omission and erasure by historians "Sappho was a man"-style or more directly with violence and burning books they didn't like)

b: it's reductive to look at pre-agrarian societies and go "ah yes, they were all X because we don't know that Y happened", absence of evidence is not evidence of absence

c: it's completely possible, even likely, that gay nomads with difficult lives had hetero sex in order to reproduce (increased chance of survival, sense of duty, furthering a family line, etc) even if it wasn't their favorite thing in the world

so I'd throw out that datapoint because it really sounds like it's starting from a conclusion and working backward to justify it instead of being an evidence-backed claim, and contradicts several other well-researched perspectives on the matter, or at least only take it to mean that the way in which we think and talk about homosexuality has changed in the past couple hundred years instead of using it to infer commonality in sexual practice between tons of disparate cultures and peoples

the way you're invoking it makes it sound like a "civilization was a mistake that made us all degenerate, we must return to monke" paleoconservative or cryptofash talking point, which, uh, fuckin' sucks, my dude