Except that then there are a surplus of males without available women, which is not good for society. Think of all those surplus Muslim young males in polygamous societies, with too much time on their hands, and not enough familial responsibilities. Thus it should not be recognized in law, and no legal benefits should attend, unless of course an offsetting number of males are truly gay. It is rather basic really. I am surprised you missed it.
Doesn't make a difference.
In our society, powerful men maintain mistresses on the side anyway.
Might as well make it legal and raise women to appreciate the benefits of having other women in the household to help with the housework and babysitting.
And at all events, it would be only a minority of men who would be able to afford more than one wife, so not that many women would be taken out of circulation.
Additionally, an increasingly large percentage of American women are not finding anyone to marry anyway.
(And no wonder: Why would men want to marry anyone when milk is free, so to speak)
One could also look at it on the flipside: Males who are successful members of society will out-breed the losers by a significant margin, improving the gene pool over the long-term. It allows women to upgrade the genes of their offspring in a fashion that would not be as easy if society enforced a one-to-one mapping. Classic sexual selection, Animal Planet style. Of course, you would probably have to legalize prostitution as a practical matter for some semblance of equilibrium.
You don't have to go to muslim societies to see the awful results of polygamy. Just look at the teen-age "lost boys" of Utah and Arizona polygamous sects driven out and dumped alongside the highway so as to eliminate competition for the women. Taxpayers pay the tab for these kids. Highly destructive stuff.
"Marriage" is defined in my 30-year-old dictionary as "an legal state instituted for the protection of women and children."