All forms of relationships will fluctuate as legal options throughout time. Polygamy is no different. Polygamy used to be common in certain parts of the developed world and is still common in places like the Middle East. Heterosexual monogamy is just the thing that it happens won’t fluctuate, this is as it’s like an axis mundi of relationships. That said, everything you describe is inevitable as a phase.
That said, I don’t consider a relationship invalid or “less valid” no matter how many people are involved, their genders, their race, their creed, their medical history, how close they are, etc.