I'm for legalizing polygamy. Not that I'm going to be having a harem anytime soon. With marriage comes a set of rights and protections under the law that wives are entitled to, but "girlfriends" and "mistresses" are not. Yet why is it legal for a married man to have a mistress but illegal for him to have a second wife? Why is it OK under the law for a married man to sneak around with a girlfriend but not OK to have another wife with the full social and legal standing as the first wife?
For a polygamy regime to work, there would have to be safeguards. There should be some limit on the number of wives. One man hoarding all the women would no doubt cause a lot of problems. IMHO, 4 is more than enough. Subsequent wives would have to have the same entitlements the first wife gets. That is, the financial support for each wife must be fairly equal. All the children from these marriages must be entitled to similar support as well.
Also, the government should not have to subsidize polygamy through welfare payments. Before marrying other wives, the husband would have to prove he has the financial means to support multiple wives and the children that would result from their union. He might even have to put a deposit with the government for each subsequent wife he marries, in the event they fall below the poverty line.