Strict monogamy and the doctrine of evolution are fellow travellers and not-so-strange bedfellows.
An unspoken presupposition in our anti-polygynous church culture (anything but polygyny!) is that, “Yes you can find polygyny in Scripture, but that was in the primitive bad old days of a proto-Christian society when the great patriarchs of old were still living in theological caves and God tolerated all kinds of pagan practices. Back then God was less sophisicated than he is now and that is why polygyny is now a sin.”
This attitude is purely evolutionary and nearly universal in the church. It assumes that God evolved in his standard of right and wrong, that His commands and laws where for back then but no longer apply. This is anti-nomianism. It also denies the Gospel since it assumes Christ came to die, not for our sins as defined by God, but for sins as we define them.
This evolutionary model applied to morality means that we must now accept homosexuality as acceptable as the next step in our evolving definition of right and wrong. After all, the same God who said said a man lying with a man as with a woman is an abomination had no problem with a man having many wives.
If God is the source of your morality then you will also believe that a man may have more than one wife at a time.