Lee Stranahan took a lot of grief in the comments section of the Huffington Post with his piece earlier this year, Why are gay marriage advocates not defending polyamory? One answer, of course, is some gay marriage proponents don’t see polyamorous marriages as legitimate in the same way they see a homosexual version of a heterosexual marriage as legitimate.
Another response is that there are already legal solutions to polyamorous marriages such as settng up chapter S LLCs, etc. This, however, strikes me as a different version of the “why can’t you be happy with domestic partnerships?”
Ultimately, however, I suspect the reason is that the fight over gay marriage is focused largely on the ‘gay’ part of the equation rather than the marriage part.
Over the past decade or so, it has become less and less acceptable — both culturally and legally — to discriminate against homosexuals. As a result, marriage really sticks out as an area where there is a bright shining litmus test that screams ‘no gays allowed’. To some extent the opponents of gay marriage are correct to the extent that they fear that allowing gay marriage will complete the project of legitimizing and destigmatizing homosexual behavior.
Polyamory, meanwhile, goes pretty much to the core of what exactly we mean by marriage at all, and why we have specific laws that treat married individuals as a separate class.
Personally, I’m all for also recognizing n-tuple marriages; if the cultures of the Old Testament could do it, surely we can too.
Update: for another take on polyamorous marriage, see Lindsay Beyerstein’s 2006 article The legal logic of polyamorous marriage.