If that's what they truly believe, then the government should ban all marriage. OK, that's a bit harsh, so let's say it should cease to recognize any religious marriage.
Reduce any religious marriage to its social components and it becomes little more than a social contract between two adults of (relatively) sound mind. That's the part the government should continue to recognize. As such, it can recognize the social contracts between any two, three, four or even seventeen people entering a civil union, whether or not those people ascribe to their union religious connotations and irrespective of the particulars of that religious coloring, if any.
It serves no purpose for the government to be involved in any religious business. Christians can keep their Christian marriage, Buddhists can retain their Buddhist marriage, and so on without the government knowing or caring a thing about it. Before the eyes of the government, which are blind to religious identity, all it sees is a social contract between two or more people.
Similarly, no religious institution need recognize a marriage with which it does not identify. It need only recognize the social contract, which grants the same benefits and responsibilities for everyone irrespective of the genders or roles or religions of the individuals involved. You might be tempted to think that the hard-line Christian churches will react to a blindfolded government by ceasing to recognize the marriages of not only gays, but of Muslims and whoever else they don't particularly like. But if history is any indication, you're more likely to see fundamentalist Christian churches failing to recognize the marriages performed by other Christian denominations, such as those that allow female priests or gay ministers to oversee marriage pronouncements.
And let them. Who cares, so long as they recognize the social contract? Are Buddhists required to recognize a bar mitzvah, whatever it may imply in an orthodox Jewish community? If a bar mitzvah does not boil down to some wider form of social contract or engagement, it is not only nothing to Buddhists and Hindus, it is ultimately nothing to Jews but a hollow ritual kept alive by habit.
Without this attitude, the government becomes de facto religious, using a religious sense to determine what is a legitimate union and what is not. I see no reason why seventeen Mormon adults can't enter into a social contract that constitutes civil union. It's not for me and I don't believe in an ounce of their religious superstitions that encourage it, but a group of people experimenting with their lives to see what makes them happy does not infringe on my social rights. They may not even articulate their actions as experiments, but that is exactly what they and all of us are doing with our lives, and we are obliged to respect the experiments with truth that others make.
Of course, if Mormon elders are tossing young boys out of their communities so that plenty of women are available for such ballooning social contracts, that's another matter; but that in and of itself does not speak to the legitimacy of any number of adult citizens of this country engaging in a social contract that results in cohabitation and shared identity.
In a free society, two or more people must be freely able to enter any social contract, so long as it respects the social rights of those in society that dutifully bear their social responsibilities. Yet it is no one's responsibility to recognize any particular religious bond, or even religious identity, directly or indirectly through taxation.
Once the government gets out of the business of consecrating one social contract over another, leaving that business to the people with vivid imaginations, everybody can live happily ever after.