It was okay in the OT, even the Law gave provisions for polygamy. Now, we as Christians aren't under the Law, there is still a model.
Yes, people who wish to be "authority" in the Church should have only one wife. The reason for that is clear. They need to focus on their tasks and duty. Having a wife is a good thing, she is a helper in many ways. A wife is a strength for many men.
Now, as society deems, it's wrong. Even among unbelievers, it's wrong. Where it came from doesn't matter. However, in America, I guess we're "free".
I've been thinking much about this lately. I have found no evidence that says it's a sin, but, have found more evidence that it's not the wisest decision. As stated before, dealing with one woman can be challenging enough, why would a man want more than one,,,,,,,,,, well, except for obvious reasons. This is true especially this day and age, where women don't seem to respect their men, men don't understand their roles, and men don't respect their women. Most people can't handle one solitary aspect of their lives outside of football, why on earth would they want to multiply tasks?
None of the concubines, nor extra wives had much of anything to do with providing stock for the world. I honestly think it's purely for then mans desire. We as men have a hard time not wanting to "hook up" with every good looking woman that winks at us. So, I believe it's more of a protection of virtue, rather than multiplication, or vanity.
I've spoken to my wife about this the past week. Just as I told her, "If I had to do it all over again, I would not have vowed to forsake all others till death do us part. I don't believe that plygamy is wrong. Given the chance, I still would've made the same choice, and married only you. I just don't like being manipulated by a man made doctrine passed on as God breathed." The GOOD woman that she is, understood.
After all that rambling, No, I don't think polygamy is a sin, I've found no evidence, (although can completely understand arguements against) that polygamy is sin.