There are species more naturally monogamous.
We're intelligent, that means we have complicated reproduction modes. The dominant and by far the least disruptive to a community is monogamy.
Followed by promiscuous men.
Followed by promiscuous women.
Something many people confuse themselves with is focusing on the sex itself. It's about the kids.
Women don't have a mode of reproduction where they don't have to invest a lot of resources. They select the gene donors carefully, but what they want more than anything is a reliable supplier of resources.
A woman can easily have more kids than she or any one man can take care of. The limiting factor is resources and our instincts reflect that.
That's why women have been perfectly happy in harems, so long as the harem male is fabulously rich.
Men can father children without investing any resources. Men in this instinctual mode only care about the sex. They don't need to admire the woman because they don't care if the children have good genes. They don't care if the woman is wealthy or if she can take care of the kids.
Civilization tries to discourage this mode. Then contraceptives made it safe for men and women. The problem is that only men have an instinctual mode that finds this satisfying, and even then it turns into a hollow satisfaction as they realize they're alone and nobody had their kids.
Women want to cheat when they think other men are superior and their resources won't be threatened. It's a lot easier when you don't have to worry about giving birth to a kid who looks nothing like the father.
Men can share, but instinct/evolution says to only share with closely related males. Instinct can be overcome but it's more likely that if men share they think of the other male as a brother.
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Calling something unnatural is almost always true in a specific context but not in another. We decided we're unnatural but that is just a convention for speaking about the cognizant vs non-cognizant range of possibilities.
Building a house is unnatural in the classic context of the word, but it isn't wrong.
Interspecies sex is natural in the classic context of the word, but - well I'm sure you know how people feel.
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I think contraceptives are great. More tools the better. I think that people should be free, and irrational condemnation of victimless sexual behavior (or relationship) is wrong.
I also think that monogamous heterosexual marriage based on genuine love (emotion which connected to admiration of genes and being a reliable asset) with 2-5 kids is the happiest human beings are going to get on average and anyone who tells kids anything else is making the world worse.
Avoid the two errors:
Conflating the unnatural with the evil
Assuming that anything less than the best is the bad, it could and often is a lesser good