You may be taking things from the wrong end.

Violence about having an exclusive mate could be a purely cultural construct, reinforced from childhood. I can say that I always, personally, considered the idea of controlling a person abhorrent, as much as I found uninteresting the idea of orbiting my intimate life around a single person with special and very high privileges over whatever I do and think, including outside of bed. However, all my relatives were very insistent that I should date girls and marry. 100% culture, 0% nature. Of course, this is just a single data point.

The heterosexuality part being the "default" is a bit naive, because it ignores the lengths to which some people go to force their kids to be heterosexual. Again, anecdotally, my father sired two gay sons, who had to go to great lengths to have a less traumatic life. Sometimes I suspect my father wasn't that hetero himself, and was only ensuring the next generation inherited his cultural legacy/trauma.

I could consider an argument that a majority (heterosexual) imposes a cultural canon on a non heterosexual minority. But the problem with that is that we don't really know if that majority/minority split would exist without the very strong cultural conditioning. And, as I said before, I don't really believe that homophobia is something the Canaanites invented out of spite. Most likely, it was a cultural trait that conferred advantages to groups, particularly after the agricultural revolution locked human population in a cycle of growth and war for land--but that's just a pet theory of mine.

There are even theories (read the controversial book "Sex at Dawn" if you want the details) that our current cultural canons about sexuality run against what was our nature for hundreds of thousands of years.

> Violence about having an exclusive mate could be a purely cultural construct, reinforced from childhood.

"Could be" is not "probably is".

All the observable evidence points to "fighting for mates being purely instinctual".

The opposite, is in fact, true: not engaging in violence for mates is, in fact, purely a cultural construct.

Culture enforces that, via laws, norms, etc.