Here's the other thing: the assumption appears to be that marriage is an inherent right, in that it is covered in "life, liberty and pursuit of happiness". The argument is that by defining for me what "marriage" is, you're limiting my ability to be happy.
So what about single people? Why can't I be "married" to my job if my job makes me happy? Why can't I be (as a children's book advocated) married to a flower, or a tree, or nature, or myself? If the judiciary can determine that marriage is essentially whatever I define it to be in order to be happy, why not?
Not to mention, it's discriminatory! The whole point of marriage being advantageous from a tax standpoint is that we want to encourage it as a society. Well, we've now decided that gay marriage and heterosexual marriage are equally effective because "it's still two loving people" - after which we quickly add that ONE loving person is just as capable, and that a child raised by a single parent is NOT at a disadvantage, because we certainly wouldn't want single mothers to feel inferior or somehow lacking. (Never mind the studies that contradict it.) So how about those of us who are single and (as far as you know) derive their happiness from being single? Why am I being persecuted because I have not bowed to the convention of marriage and made myself less happy?
This is what happens when the government starts to try and interpret "pursuit of happiness" as "legislate people INTO happiness" as opposed to interpreting it as "if we leave people alone, they will pursue the things that make them happy."