Monogamy vs Non-Monogamy

Not all couples subscribe to monogamy. There is no reason to be judgemental about this in the absence of other information.

Marriage != monogamous relationships

Contrary to popular belief, monogamy is not the only way to have relationships.

Relationships between people that aren't the same age aren't exactly unheard of, and exclusive monogamy isn't exactly an ironclad rule among humans, so two key assumptions underlying your conclusion seem to be invalid.

One problem: humans, even in western societies, aren't strictly monogamous. Men and women both can, and not infrequently do, have more than one partner, even when married.

Ignores the apparent fact that polygamy and rapid serial monogamy are very common in under-25's but happens a lot less with married people. So young people have a lot more mates available by sharing.

Some skepticism is fair, but notice that - because non-monogamous relationships are somewhat socially unacceptable - we'll perceive them to be less common than they are, because people don't talk about them. So randomwalker's situation might be more common than you might think.

Seems pretty self-explanatory.In most places in the world, we are raised to pursue monogamous relationships, whether that is predicated on the idea of "true love", or just the need to "settle down and start a family". Either way - monogamy.In reality, a lot of people aren't happy with monogamy. See the extreme prevalence of cheating/divorce. I'm not OP, but I think the world would be a much happier place if everyone stopped trying to be so monogamous all

Guess: when you're looking at one good-looking woman you're thinking monogamy. when you're looking at two good-looking women you're thinking polyamory. ;)

makes one wonder if having an affair/fwb is the common and cold reality behind the curtain of the monogamous relationships.