surah an-Nisa 4:3 - " If ye fear that ye shall not be able to deal justly with the orphans, Marry women of your choice, Two or three or four; but if ye fear that ye shall not be able to deal justly (with them), then only one, or (a captive) that your right hands possess, that will be more suitable, to prevent you from doing injustice."
surah an-Nisa 4:129 - " Ye are never able to be fair and just as between women, even if it is your ardent desire: But turn not away (from a woman) altogether, so as to leave her (as it were) hanging (in the air). If ye come to a friendly understanding, and practise self-restraint, Allah is Oft-forgiving, Most Merciful."
This'll be quick. Historically, many societies were polygamous. Specifically, polygynous, meaning that men were allowed to have more than one wife. We have examples of this in the OT. Jacob had more than one wife - Rachel and Leah, as well as their handmaidens. Whether or not he 'married' the maids, he was permitted to have sex with them and acknowledged his children by them. David, Solomon, yadda.
And while modern society has shifted to monogamy, it's a cultural change. There is, to my knowledge, no explicit teaching in the Bible that says, one man, one woman. There are plenty of things that are interpreted that way, but that's a matter of interpretation. There do exist Christian (and I'm not talking about the Mormons. I don't count them.) polygamists. And they have their own interpretations. So let's not get all judgy on the Muslims here, okay?
The Qur'an, here, *limits* the number of wives that a Muslim man can take. He puts a cap on it at four. Prior to this, there was no such limit. They could marry as many as they liked. But there are more conditions placed on it than just the number. A Muslim man is only supposed to marry more than one woman if he can be equal to all of his wives in all ways. Equal support, equal affection. No favorites. They must all be treated exactly the same by him. Which, as is further pointed out in the next quoted verse, is impossible.
It seems to be another one of the instances where a limit is placed on a pre-existing practice, even as it is shown that the practice is not actually preferred. Change needs to be made slowly for it to take root. You can't just say, 'no!' and not have backlash. You have to change the thoughts of the people before you can change the practices. Considering that polygyny is still practiced in Islam, clearly this wasn't entirely done away with, but it was changed, adapted from what came before.
I really don't have an issue with polygamy. It's not for everyone, obviously. Some of us don't share well *raises hand*. But it does work for some, and as long as everyone involved is an adult who made the decision informed and freely, I've got no issues. Unless, like some, they are defrauding the government, claiming to be single women in order to receive aid. But that I tend to hear about with Mormon's more than Muslims.