Honestly I thought it was because of cultural differences in the way stories are told.
I would think if they are spending time to create something they would actually want to capture the audience instead of making subpar stuff. Especially if they decide to translate them to other languages…
The frowning upon is religion based values. On the other hand multipe wives and no other husbands allowed is also religion based.
Next question would be why is same sex marriage frowned upon?
There is a reason why ancient societies were matriarchy based. It was always easy to tell who the mother was. And because of that the wealth was tied to the mother’s bloodline instead of the father’s.
If you want a real - future proof inheritance system: leave everything to the mother than as she dies the children get everything.
One on one pairing is more fair with big numbers. You no longer have to take your brother’s widow as a wife, you do not need to marry to make sure you have one extra goat to milk ar having a maid and incubator for the next generation.
Humans generally speaking do not need to make more than 2 kids. Your bloodline won’t die out because of modern medicine. Nobody’s bloodline worth anything anyway. Your genes are not that superior.
So there is no logical reason to support a one sided marriage system.
Now open polyamory where everyone is on board, equal, open about it and can leave any time - go ahead that totally green light from me.