Heard about this on the radio last night: a Utah triad is challenging the ban on polygamy.
As Scalia (?) predicted, it was bound to happen sooner or later. I, of course, being timid and afraid of rocking the boat, think later would be better.
The reason I think that is that same-sex marriage (which is mighty controversial in its own right) is always cited as a gateway drug to polygamy and bestiality. Pretty much every anti-gay-marriage screed ends up saying something like "And if we legalize this, what's next? Polygamy? Bestiality?" It's used as a reductio ad absurdum, as something so obviously bad and wrong that all reasonable people will see what a bad idea gay marriage is. And that seems to be an effective argument, as court decisions have said things like (paraphrasing) (I need a quotation-mark-like symbol for paraphrases) "Despite the opponents' fears, there's no danger of this decision leading to legalizing polygamy, because marriage can obviously consist only of two people."
(See this Boston Globe opinion piece, for example: it starts out as a clear and detailed look at the case in Utah, then it descends into ranting about how this is obviously what comes of legalizing same-sex marriage, so "the Massachusetts Legislature [should] reclaim its authority to say what marriage means [and not] meekly submit to the high court's power play.")
In other words, just as it took some time between legalizing mixed-race marriages and legalizing same-sex marriages (and I'm not convinced that the latter fight will be won anytime soon), I think it'll take some time between widespread societal acceptance of same-sex marriage and willingness to even think about polygamous marriage. I think it's a necessary fight; I just don't think it's one the country is ready to consider yet.
But I may well be much too timid. (And it's easy for me to say it's not urgent; I'm not being disenfranchised, 'cause I don't have any plans to get married at all, regardless of the number of people involved.) If I'd been around for Birmingham or Selma, would I have said "The South isn't ready for black people to be given equal rights, give it time"? I hope not, but it's hard to say. In retrospect, I would say the civil rights people clearly and obviously did the right thing; maybe a few years down the line I'll realize that change needs impetus. Dr. King, who would have been 75 this past Thursday, wrote from Birmingham jail: "I am in Birmingham because injustice is here." (And no, I don't think that the illegality of poly marriage is really comparable to the civil rights situation in the South in 1963. But it is nonetheless, I feel, an injustice.)
As for bestiality, for my money a marriage should be contract, entered into by consenting adults. Since I'm not yet convinced that nonhuman Terran animals can give full consent, I'm dubious about the idea of legally allowing marriage with one. But at such point as we meet sentient aliens, I do think humans should be legally allowed to marry them.
It should also be noted that polygamy as practiced in some parts of Utah doesn't qualify as marriage by the above standard, in that some of the participants are twelve years old, which is currently below the age of consent (may not be work-safe, especially due to ad at the top of the page) in all US states. Regardless of age of consent, I feel that coercing anyone into marriage is a bad idea. But that's a separate issue from whether multiple consenting adults should be allowed to enter into state-sanctioned marriage with each other.