Come to think of it, let us disregard constitutions and laws for a moment. I would be interested to hear a single robust philosophical argument (as opposed to a merely practical one like "there are a lot of same-sex couples, and it would be more efficient to allow them to have visitation rights, etc.") that anyone can make for allowing same-sex marriage that is not also an argument for allowing polygamous marriage.
Sure, but you won't like it, because it relies on fuzzy cultural markers instead of absolutes.
The California Supreme Court recognized a legal space for same-sex couples because there are hundreds of thousands of such couples across the country, setting up households, many of us raising children, and interacting with the institutions of government in a normalizing way.
There are lots of individuals who don't like gays and try not to think about our relationships, but it has become a matter of legal stature not to harass or outlaw same-sex couples setting up households and a question of professionalism to at least defer to our relationships in many forms where it is not a legal question, certainly in California. There are hundreds of thousands of us with mortgages, children, health care arrangements, and other totems of couplehood that are legally recognized and bind us publicly and privately as couples in dedicated, loving relationships.
You see, this issue is one for dorm-room philosophizing (or better, nursing-home philosophizing) only if same-sex couples are an abstraction or a rarity, like an animal with two heads. For many Americans, we are. Others deal with us on a daily basis, but some of them lack imagination or are just mean when it comes to viewing us as equals. (Ha, won't that statement get a response.) However, in the aggregate, we aren't, which is why decades after gay couples started coming out of the closet and adopting conservative forms of living, we are being recognized.
Not only does my partner have legal rights stemming from our relationship, even pre-marriage, but he has a social identity as my partner that we would never have if we were in an incestuous or polygamous relationship. He comes to social events with me at work. Our families introduce us to others. We go to charity benefits together. I've got a picture up at work. Not all gay relationships can do this, but no incestuous relationship can.
If polygamous families, let alone incestous families, ever achieved the numbers, diversity, and public visibility in their relationships that same-sex couples had, they might be in a place where they could make that argument. Suffice to say, though, just as the Supreme Court could uphold sodomy laws in 1986, they could argue there is no compelling reason to recognize relationships that do not exist in the eyes of the vast majority of Americans and which have no tolerated legal standing or place in our society. As opposed to same-sex couples, who may not be in leadership positions in Southern Baptist Churches or leading national tickets, but who have achieved a degree of visibility and integration that people deny only from a position of ignorance or limited experience.
