From a philosophy prof reader:

Dear Jonah,
I am surprised that critics of Same-Sex Marriage, while adopting the Reductio Ad Absurdum argument of Polygamy, don’t go the next step and discuss how polygamy, as actually practiced in some parts of the world, as a matter of fact does promote social and economic stability.
I lived for three years in a very small village in Sierra Leone in the late 1970’s and can attest–as can many of NRO readers who have similar experiences–that polygamy has a rational basis in poor, agrarian and autonomous regions of the world, where only half of all offspring live to their 5th birthday, where there is a strong need for children to work the fields and, in later years, to care for dying parents, and where there is no social safety net, let along opportunities for employment for young women, to cushion the blows of illness and old age.
The point is, Polygamy does have a rational basis, even if on occasion the multiple wives may bicker among themselves and with the husband. So if the proponents of Same-Sex Marriage consider stability a virtue, then they should also be in favor of polygamy. The only reason for Rep. Frank et al. to oppose it is to suggest that monogamy is culturally superior. But they wouldn’t want to say THAT, would they?