I believe the basic problem with this whole idea is the fact that the large majority of people equate the term "marriage" with sexual intercourse. So let's, for the sake of argument, relegate the term "marriage" to the religious dust bin, and redefine the joining of two or more people into a private financial and emotional relationship as a civil union. No ceremony required, no limitations on sex or numbers, no religious connotations at all. Basically, the same kind of thing which happens when corporations merge, a legal bonding of the group for mutually beneficial reasons.

Any heterosexual couple wanting to be married, whether in a religious environment or not, is required by law to get a license. This license makes their union a legal one, whether they go through the ceremony or not. As such, they are entitled to certain benefits, including health care and taxes, which the law permits such unions.

As voxelectronica has pointed out, the government has no business in our bedrooms. If the members of such a civil union enjoy sexual relations, it is their business, only. If they happen to be members of a religious community and that community opposes their actions, that community has the right to bar them from the community. They do not have the right to dissolve their union, except as a prerequisite for membership in said community.

Regardless of how you want to interpret the Constitution, US law permits such unions between a man and a woman. Denying this to a couple simply because they happen to have the same type of genitalia is absurd. This is a business decision, not a medical one. By the same token, if two men and a woman want to form a union, or two women and a man, or ten women and a man, or even ten men with one woman, the government should have no legal grounds to forbid such a union. (I can't seem to find, among religious groups which permit polygamy, any that allow one woman to have many husbands. It always seems to involve one man with many wives. Interesting. Another thread, perhaps.)

In a perfect world.

And to voxelectronica: it may be true that the Constitution does not specify marriage as a fundamental right. But neither does it "permit" the use of CD players, cell phones or disposable diapers. What's important is that the Constitution does not prohibit these things.