Probably if everyone in the United States circa 1960 had known that taking modest steps in the direction of feminism would, in fact, lead during their lifetimes to the legalization of sodomy, to gay men marrying each other, to a small but growing number of fathers staying home to take care of the kids, to legal abortions, etc., etc., etc. the public woud have overwhelmingly rejected those early steps. But the poo-pooers won the day, the people did not believe, and now majorities support most of those developments, and all signs are that the unpopular cause of gay marriage will grow more popular after some generational turnover.
Now I think it's just great that the slope has slipped as far as it does, and hope it will slip more. So I have mixed feelings about the pragmatic political necessity of convincing people that the slope will not, in fact, slip. But it seems to me that gay marriage probably will lead -- not as a matter of metaphysical certainty, but just as a matter of banal causal fact -- to some kind of legal recognition of polyamorous relationships at some point down the road. And I think that's fine.
Hat tip: Sara Butler.