Yeah, no, I agreed with you on the other things, but then you had to post this. People are naturally polyamorous, it was culture and religion that forced monogamous relationships and marriage.
I agree people are naturally polyamorous and a stuff ton of marriages either fail or have at least 1 temptation of cheating.
However people get married with the belief that their spouse will remain faithful forever "till death do us part" so they believe when they get married this ensures them a safety net of constant love and children popping out.
That's the only reason why marriage exists. However this is all a lie, never get married.