The reality is that people are biased towards their own biological children (on average), and men would in generally really prefer to raise their own child, rather than someone else's (on average). Raising a child requires an immense amount of resources.

If you presume that men who discover that their partner have slept around, and their child is not theirs, and worse, they've sunk like 3 years of resources in keeping this kid alive, are even just slightly more likely to be liabilities to society, then you could see why such a societal strategy might emerge.

The converse situation (men sleeping around) is somewhat different (though obviously related). The reality is that if you want to retain an approximately balanced gender balance in a population (perhaps because uh... you might need to compete in raiding/warfare), then you -cannot- have even a significant minority of men having exclusive access to more than one female without having significant societal stability issues. Combine this with men like knowing that their child is theirs, quickly settles you into "monogamous-ish".

And it's "ish" because there are always exceptions.