I think itīs cultural teaching.
The ancient Greeks accepted homosexuality (between men, I donīt know about the women) as a fact of daily life.

Kings had harems. I doubt they only loved / felt great affection for only their queen / main wife.

There were tribes of natives, e. g. American natives, where a man could marry more than one woman. This was accepted there, too.

In the Muslim world or with the Mormons, a man is allowed to marry more than one woman.

(I cannot help but wonder why there is no model of a one-woman-several-men marriage anywhere, but nevermind.)

I wish to point out that if, once you are in love, you were not attracted to anyone else anymore, there would be far less marital trouble and divorces.

So I think the one-on-one model is cultural, and must not be seen as "the only normal/right thing".

Nevertheless, I am - in a relationship with a man - a jealous bitch and would not want him to have several subs or girlfriends.
However, knowing people are no saints (and having had a strong attraction to someone else earlier in life in my only other serious relationship with a man before my current one, though I had felt 100 % in love and therefore 100 % safe from temptation), I told him that if he ever felt the need to stick his cock elsewhere for a night, I would not want to know and expect him to go about it in a way that would make it unlikely that I found out.
I would feel it as "broken trust" and donīt think I could handle it if I knew.
No, I would not hate him for having fancied another girl, I do know that can happen, and it does not mean you are not good enough anymore or boring to him - but ever after I would wonder if he really was at home, really on a business trip, really wherever he said - and not with some other chick? I cannot live with broken trust.
But Iīd rather have him having a one-night-thingie than have him sit at home and radiating he would rather be elsewhere - I am too proud for that.

During my teenage years, I had one relationship with a lady. She is still my favourite lady. We are both bi (half bi; we both like the upper half of women and donīt care overmuch for the lower). With her, I did not care if she had a man for a night, or some other girl. I did not feel hurt, it did not make me feel I had been cheated on, or made me want to sleep with someone else too to equal the balance.
Those casual things had nothing to do with us.

This is also why I believe it is just a matter of cultural imprinting. No one taught me how to lead a relationship with a woman. No one taught me if a woman I was with had a fling with someone else it was "evil" and "cheating" or "it is a sign I am not good enough". We were always open about it. The only point was that none of us started a relationship / committed herself to somebody else - in this we were exclusive.

Though I am only upper-half bi, I replied to my hubbyīs question to tell him freely and uninhibited what would be the ideal relationship for me, that the ideal for me would be to be living with him and her in a house so I could have them both anytime (yes, she is still in my heart, yes, it is possible to love more than one person, and no, hubby does not mind that. He would mind it if it was another man, though).
He was stunned (apparently he expected to hear something else).
He even was allowed to kiss her when we were all out together, and I did not mind.
With any other bitch, I would have ripped his head off *g*.

But - the lady in question wants to be the alpha bitch in a relationship with a man. So do I. Thus my dream arrangement canīt come true.

As for the polyamorous "trend" that came up in my area lately, particularly in the BDSM scene: I do not think we are ready for that for the above-stated cultural reasons.

It might help if I had seen one single example of such a relationship work well without any of the partners feeling they did not get enough... but that has not happened yet.

Apart from that: I think everybody should seek happiness in a way that suits him best. Other peopleīs love lives are their concern and decision, not mine.

Kind regards
Arria