As I said in another thread, since marriage has historically and traditionally a religious activity, legally sanctioned by the state, then marriage should be correctly defined as sacramental in nature, and not legalistic in nature.
No one has a constitutional "right" to receive Holy Communion. We have a constitutional right to attend and hold membership in a religious organization in which Holy Communion is offered. But no one has the "right" to receive it. Who does and does not receive Holy Communion is left to the discernment and discretion of the Church authority.
Similarly, no one (monogamous heterosexuals included) has the constitutional "right" to be married. Marriage never appears in the First Amendment to the Constitution. Marriage has traditionally been the purview of the Church. We have the constitutional right to attend and hold membership in a religious organization in which marriage is offered. But no one has the "right" to participate in it. Who does and does not marry should be left to the discernment and discretion of the Church authority.
Non-religious people have the opportunity to go to a civil authority and be "legally married." But I'm not so sure that that kind of arrangement could be strictly defined as a "marriage." A "civil union," certainly. But marriage implies an act of God, and an act of God cannot be legislated by this government, nor can the government grant access to an act of God as a constitutional "right."