Actually, nothing is over until God says it's over. At least, this is my understanding of how religion operates. So though I don't want to accuse Mormons of being a bunch of serial quitters, let's just say this isn't the first time they've folded in the face of adversity. Luckily, every time a door closes...
Obviously, polygamy, once practiced by early church members, has long since been renounced by the Latter-day Saints. In the late 19th century, after decades of religious persecution and pressure from the U.S. government, Mormons dropped the practice (although some obstinate fundamentalist groups persist). The LDS church is probably the last group willing to bring up plural marriage, but now would be the time to right some historical wrongs.
The would-be polygamist needs only to use the arguments of an advocate of same-sex marriage: What kind of moral claim, for instance, does anyone have to stand in the way of peaceful, consenting adults who call their union a marriage? Shouldn't all minorities, no matter how beleaguered, be able to enjoy happiness with the one -- or ones -- they love? In a perfect nation, wouldn't all Americans be immune from the cultural and religious prejudices of others? Isn't it a tad judgmental for us to force every household to comport to our own stifling definition of family?
After all, how does another person's connubial situation undermine your marriage or corrode the institution anyway? Have you seen the divorce rate?! Straight people have already ruined the institution. And we want in.
Personally, I don't have any misgivings about same-sex marriage, mostly because I don't believe that it destabilizes society or family -- although I suspect that it will offer gay Americans far more stability. The policy debate is about over anyway. These days, we should be more troubled by the persistent need to coerce and demean those who hold religious objections to gay unions into compliance. And if I were, say, a practicing Catholic, I could never accept that the sacrament of marriage could be redefined by judges, democracy or anyone other than the Big Guy. This is not homophobic, nor does it undermine your happiness. In my libertarian utopia, people are free to enter into voluntary arrangements, and others are free to believe that the participants of these arrangements will suffer eternal damnation.
The polygamy argument offends many advocates of same-sex marriage, who view it as an unfair and unnecessary distraction. But I'm not offering Rick Santorum-style slippery slope arguments here. I don't believe polygamy will be legalized. My sense is that the vast majority of Americans have little interest in entering into a menage a trois (well, in its literal meaning, "a household of three," at least). Rather, I'm asking: On what logical grounds can a person argue that gay marriage is OK but polygamy -- or any other type of marriage -- is not? If your answer is an arbitrary declaration -- such as, "The ideal union is between only two individuals" -- then all you've done is redefine the parameters of marriage. You support gay marriage, not "marriage equality."