Here's the thing about marriage in general - there are two components to it - a religious aspect and a civil aspect. Actually, Jewish marriages are more "legal" than they are religious, but the laws are part of the religion, so it gets confusing.
Orthodox Judaism forbids any number of marriages - divorced women and Kohanim (the priestly caste), converts and Kohanim, second marriages for a husband and wife where the wife had another husband in between. Jews and nonJews. These are the laws of the religion, and I have no problems with that - I feel for the people who can't get married under Jewish law. I especially feel for Kohanim who realize after the divorce that it was a mistake, because a Kohen can't even marry his own divorcee. But these are the laws and Gd has reasons for them, and I don't have to like them.
All of these marriages are permitted under US civil law, and I wouldn't want it any other way. Civil marriage is a contract - a shortcut for a huge number of rights and benefits, such as inheritance, medical decisions and property rights. There are so many of these that it's impossible to duplicate them any other way and all those duplications that can be done can also be challenged by family. And none of these have anything to do with religion. Yes, the Jewish ketubah details some of these, but it's a decidedly one-sided documents (all the rights are the woman's, all the responsibilities are the husband's.) and it just shows how much civil law there is in Judaism. It's also irrelevent.
There is no reason under civil law why marriages should be restricted to heterosexual couples, or couples at all. I can't seeing it destroying the fabric of our nation if three people decide to make a legal commitment to each other. This would not require Jewish law, or Catholic law, or any Protestant church, to recognize these marriages. There are already marriages that the first two do not recognize, such as the ones I've listed above.
What makes the whole thing a marriage, so far as I'm concerned, is that there should be a couple of hoops to enter it - a license, at the very least - and many, many hoops to dissolve it. There should be thought in both entering it and leaving it, but it should be possible to leave it.
No one will ever compell a religion to recognize a marriage that goes against its principles, and that's fine. It has nothing to do with civil marriage.