I think this is the crux of the issue:

"In his testimony, Justin argued that polygamy is the foundation of faith in Africa and warned about the risk of alienation should the Catholic Church insist on monogamy.

“Belief is a religion; religion on the other hand is culture, and our culture here in Africa is polygamy,” the father of 12 said, and added, “As Africans, we believe that polygamy is wealth and a source of blessings.” "

I'm sure that the Aztecs would have said that human sacrifice is a "source of blessings," too, and that slave holders in the Ante Bellum south would have said slaves are a source of "wealth."

And the Romans and Greeks would have definitely said that men having sexual access to anything they could claim ownership was their "culture."

If being a Christian means being counter-cultural, I know what the obvious Christian reaction to polygamy is.