There is a real phenomenon where many people try polyamory before they accept that their original relationship should end, then go back to just being single or start a different monogamous relationship.
This "transitional" polyamory is often looked down on but I think it's another honest attempt to deal with the pressures and problems of expected monogamy.
My friend did it. Initially it was because her SO became basically asexual and she was trying to make it work while also meeting her own needs, and she ended up leaving him for her polyamorous partner and they got married and have been together for ages and had a baby. Sometimes the way on is the way out I guess.
And this is exactly why obsessive polygamy monogamy is actually pretty toxic when you think about it. This kind of experience could also reasonably lead back to your partner, along with a renewed sense of dedication, if such a lapse of judgement was tolerated the way basically every other misstep in a marriage or serious relationship is tolerated.
I see couples forgive way worse shit than a bit of meaningless infidelity on a routine basis.
Do you mean people who feel strongly about having multiple wives or husbands? People who have many previous marriages? People who obsessively collect spouses? 😄