An interesting question:
Jonah…Why is polygamy further down the slippery slope than same-sex marriage? Polygamy has actually been practiced in the United States before and is still practiced in much of the world. Same-sex unions of any kind have never had any legal recognition in any part of the world (though some societies have clearly been more tolerant of homosexuality –which has inevitably lead to pederasty in those societies). Further, polygamous marriages are still procreative relationships and fulfill the ultimate goal of marriage laws–a link between procreation and child-rearing. Same-sex relationships are inherently non-procreative.