system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't I like the idea of my partner sleeping with other people than me - Is it cultural or natural?

I am aware that some like the idea of sharing their partner with other people, however, most couples, married or not, seems to solely rely on each other when it comes to having sex. But why is that? Is it our nature and instincts that tells us to stick with one person at a time, or is that just our culture, maybe originally derived from religion?
gpt
It's a bit of both. Western culture puts extreme emphasis on sexual fidelity not found in other cultures, but a certain amount of jealousy is natural as well. 

Humans did not evolve in nuclear families. The Stone Age was not like "The Flintstones". The general culture schema in which humans evolved was in bands of 20-100 nomadic hunter-gatherers. Different such tribes are different but the general trend seems to be a moderate amount of pairing together within an overall environment of sharing everything including sex. 

There was no issue (as others have and will try to tell you) of "bringing home food to children not your own" because all members of the band share in the food anyway. No matter what you bring home it will be shared by everyone, so you will always be feeding children not your own (but still probably closely related to you because they're on the same band as you, so evolution smiles on this).

Because children were generally supported communally by the band, the issue of raising children not your own is a null one. All the children are yours and you supported them all. In fact evidence of strong sperm competition in our biology and hidden periods of female fertility indicates that at least some females were having sex with the entire band, giving every male reason to support her children. And of course males were sleeping around too. 

Anyway the upshot seems to be that there was a lot of monogamy in our past, and a lot of nonmonogamy too. A moderate amount of jealousy is biological, but the trope of undefeatable extreme romantic jealousy is cultural. Social monogamy is nature, but strict sexual monogamy is nurture.