@shortstories @eastreed Good points. One of the ideas regarding polygyny under the manorial system was that a man could keep a wife and then a harem of concubines or wives to have lots of kids.
The West rejected this because it resulted in too many single men without a hope of having wives. The East did not.
Donations are the lowest risk. If you have a high-status high-wealth man, he can sponsor someone else's wife to have an extra kid that he funds, allowing that wife to also have more money for her other kids.
I think birth rates are 51% female to 49% male, is that correct in your recollection?