Changes to Monogamy... I'm scared

Monogamy is, at least in my mind, broken down into two categories... sexual monogamy and social monogamy, with the social monogamy being couples in a shared relationship with one another (married or otherwise), often with offspring. Social monogamy does not necessarily mean sexual monogamy. At least that's what statistics tell us. From what I've read and seen, this has held true throughout the history of man.

As for the legality of polygamy, I think it's legal in only 5 or 6 countries and then only if you are of a particular religious belief. Most if not all Western nations have outlawed polygamy and there isn't any push for it to become legal.

Consensually open marriages come in at between 1 and 4% of couples (again, depending on where you live), with a widely accepted failure rate of about 92% - so it isn't a thing, not really, except for a tiny fraction of couples. Non-consensual open marriages (one spouse is unaware of the other having sex outside the marriage/relationship), or what is more commonly called cheating, constitutes a much higher percentage of couples, again often resulting in the failure of the marriage/relationship.

So while cheating happens in a fairly high percentage of marriages/relationships, it isn't morally acceptable and for sure is not legal in most countries.

I don't think cheating, polygamy or open relationships are on the upswing, it's probably more that we hear of it more today than we did a few years ago.