I think this is a simplistic understanding of how “nature" works.
Nothing in biological nature is the same 100% of the time. It’s natural to be heterosexual, because that’s how breeding works in mammals. But that doesn’t mean homosexuality doesn’t exist too, in smaller numbers. It’s natural to be sexually dimorphous, but that doesn’t mean that intersex people don’t exist. Variation is natural. It’s a part of evolution.
That becomes even more true when you look at something complex like human emotions. Many, many scientific studies and anecdotal experience have shown that testosterone affects emotion, causing higher incidence of anger and physical aggression. Trans men who go on T have documented this, as have trans women who started taking estrogen and felt less anger than they did before.
Likewise with monogamy, it may be “natural" to form a stable relationship for childrearing but at the same time it’s also “natural" to be attracted to multiple people. Monogamy is a choice not an orientation. Monogamous people are very capable of finding other people attractive they just don’t act on it (unless they cheat).
Social structures aren’t created like “mwahaha let’s enforce this unnatural thing that’s never happened before because it suits me". It’s more organic and happens slowly over time. If something is more common like monogamy and heterosexuality, insular communities will just think “what’s more common is natural" and then deviance from that can be seen as threatening. Not to mention cultures before birth control or STD protection, there is a motivation to dissuade people from actions that could cause unplanned pregnancies or venereal diseases. Waiting for sex until marriage isn’t natural but it was a way for people to avoid pregnancies and STDs.
Social norms usually develop from natural tendencies in the majority and are then codified in tradition and passed down. This can become very strict and oppressive if the natural human propensity for difference is ignored and demonised.
But that doesn’t mean it’s not “natural" for men to be more aggressive, or for most people to choose monogamy, or for most people to be attracted to the opposite sex or for most people to be sexually dimorphous.
At the same time, deviation from these norms is also natural.
