Dating several people
I mentioned several possibilities, but spent quite a bit of time discussing the evolutionary perspective because there has been a lot of research in this area and because it was an issue raised by the reader who submitted the question.
To address the specific points you made in your post, I should first say that my statement “research from around the world…” was not an exaggeration.
This probably stems from the fact that marriage (which is almost universally defined as a union of two people) is frequently promoted as the “ideal” relationship state we should all be striving for.
As a result, it tends to be the exception rather than the rule that people report engaging in non-monogamy.
But again, forgetting how modern conventions have impacted this, have you ever heard of something called birth control?
Women who keep getting impregnated by guys who fail to stick around and help out with the kids may find themselves at an evolutionary disadvantage because they have to provide for themselves and their children entirely on their own (which probably proved exceptionally challenging in the days of hunter-gathers).
They want more partners for short term relationships and the same social stigma for women being sexual is not an issue.
You also back up the rusty stereotype that 'by nature' men want to spread their genes.
The specific article I pointed to as support for this idea (Schmitt et al., 2003) included samples drawn from six different continents and 52 different nations.
While that does not necessarily represent every single group of people in the entire world (which no scientific research ever does), it is about as close to a worldwide sample as I have seen in an academic paper.
In the article you say, "Research from around the world has shown that men have significantly greater desire for “sexual variety” (i.e., having a lot of different partners) over the course of their lives than women." First of all, this comment is way too general.