The reason men had more than one wife "back in the day" is because there were always more women than men. A woman might get married and have children, then her husband gets killed in a war. Now she has no way to care for her children but to become a prostitute. So, they allowed men who could afford to have extra wives (In Islam I believe the limit was(is) 4, I don't know about other cultures)...they had to be able to support them all (usually a different house, or at least a seperate area in a large estate for each wife), had to treat each fairly, and weren't allowed to engage in sexual relations with more than one wife each day.
It's in a male's nature to be polygamous also...a man can have 4 different kids with 4 different women all at once (while a woman, of course, can only bear one man's child at a time). Just a faster way to populate the earth, I guess.
I really don't think this is a valid argument in today's society, where women are allowed to work. and the earth is overpopulated enough already...no need to go around spreading your seed like crazy. as far as 2 men, one woman, or any combination because they're "in love" with each other and so forth...I don't think that's right...but I also agree that it's not the government's place to say you can't do it.