Well, we’re not supposed to look at history, or think about whether polygamy or homosexual marriage or polyamory or any of that is good for society. We’re in an age where we say that consenting adults have virtually unlimited “rights” to do whatever the heck they want.
And, with the move towards homosexual marriage and possibly polygamy, we’re also saying that society and culture and government have to officially endorse whatever it is that these people want to do.
So while you make a good point about how you would have large numbers of men with no wives, and what impact would that have on society, the liberal thought process does not allow us to go there. Instead, the desires of the consenting adults who want to be polygamous is paramount.
I don't see that as a problem, in fact, I know many right now who would gladly give theirs away to the first taker.