As a person of faith, marriage means a lot more to me than how it is defined by the state. It has a spiritual and personal dimension over which the state has no authority and in which the state should have no interest. The state does not legitimize my marriage; rather, it demeans it by conflating its concept of marriage with the totality of Marriage as experienced by couples.
The state imposes burdens and grants benefits to married couples under its licensing scheme that are unavailable to unmarried couples or that are difficult for unmarried couples to achieve through private arrangements. Why does the state confer these benefits on married people? What state interest, if any, is furthered by the licensing scheme and the exercise of jurisdiction over marriage? Based on the arguments of would be defenders of traditional marriage, I imagine that the state reckons that it should promote and reward relatively stable domestic arrangements. These are supposed to be conducive to the public order, the rearing of well adjusted and healthy children, and the welfare of the participants. Married people are said to prosper.
Conceding for the sake of argument that this is a legitimate state interest, why would the state limit the benefits and burdens of marriage to heterosexual couples? Wouldn't any couple (or polygamous group for that matter) benefit from stable domestic arrangements just as much as heterosexual couples, especially if they are rearing children? Wouldn't the interests of the state be served just as much by conferring the benefits and burdens of state sanctioned marriage on anyone who seeks them without regard to gender or sexual activity?
Suppose a brother and his widowed sister want to live together and raise her children together. Allowing them to form a civil union, despite the nonsexual nature of their relationship, would be a benefit to them and would advance the interests of the state. Suppose two friends of the same sex decide to pool their resources and establish a household on a long term basis. How would denying them the chance to form a civil union further the aims of the state?
Whether the proposed couple is sexually active should be a matter of indifference to the state. Stable domestic arrangements aren't really about sex, as anyone who has been married for a long time can attest.
I would like to see the state get out of the business of regulating domestic arrangements altogether, but short of that I would like to see the term "marriage" replaced by "civil union" or some such thing as Sunstein and Thaler suggest in Nudge. That way, churches can be free to define marriage any way they like, and the state can treat all its subjects equally.