Asklemmy

we're heading toward monogamy at the main relationship status. Biologicaly it the best model for offspring survival

In the ethical nonmonogamy (ENM) circles, the form of polygamy is usually frowned as it is a form of power over others. However polyamory and other forms of non monogamy are much practiced and common.

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.

I think you are talking about marriage and family, more than just sex, right? Because sex-wise, you can do what you want already.

Polygamy no. I don't think that's what most people want, the sister wives thing. That's a system used when men are scarce and you are trying to increase the population quickly, neither of those conditions exist now, and polygamist systems are often dead patriarchal and nasty.

Polyamory? The make your own family, whatever configuration, more than 2 people? I think we are closer to that, yes. In a time when you are trying to decrease birth rates, yes families with more than just a couple might become popular. More parents to love and care for each child would be handy.

Polyandry, two or more husbands? That would work in a world where there were more men than women - but most of those places in the world right now are not places where a woman would have the freedom to do that.

Monogamy has never been the main thing. However, with the equalizing of sexes in marriage according to the law, I don't see how anything but monogamy can be legally until a lot of work is put into defining how three equal people can be married.

A form of polygamy is available to the upper class; it is called having a mistress. However, the mistress has no marriage rights; any rights would come from being the parent of a joint child.