So perhaps you've seen this on the blog of the Friendly Atheist, the video clip of Rick Santorum comparing gay marriage to plural marriage?
First, I'll point out that polyamorous marriage is not legally the same as a "binary" marriage. The law in most societies is set up already to easily handle marriage between a couple - no matter the sex. There are no legal complexities in a marriage of two people. While the legal complexity of a union of three or more people can get complicated very quickly.
But I still find that I'm in agreement with Rick Santorum in that I see no ethical reason to restrict a legal union to an arbitrary number of two people.
Don't get me wrong, Santorum is being pretty smug in this video where he thinks he's cornered the questioner with a foolproof "gotcha". But even a stopped (analog) clock can be right twice a day. And Santorum is right here, when it comes to ethics.
If it is ethically okay for two people to get married, no matter their gender, then it is okay for three or more to get married. This is an argument that works even without considering same-sex marriage. Bringing gender into this argument is unnecessary, and may be a Red Herring fallacy.
(1) the fact that a State’s governing majority has traditionally viewed a particular practice as immoral is not a sufﬁcient reason for upholding a law prohibiting the practice,
(2) individual decisions concerning the intimacies of physical relationships, even when not intended to produce offspring, are a form of “liberty” protected by due process.
This case does not involve minors, persons who might be injured or coerced, those who might not easily refuse consent, or public conduct or prostitution. It does involve two adults who, with full and mutual consent, engaged in sexual practices common to a homosexual lifestyle. Petitioners’ right to liberty under the Due Process Clause gives them the full right to engage in private conduct without government intervention.
The Texas statute furthers no legitimate state interest which can justify its intrusion into the individual’s personal and private life.
If two adults, "with full and mutual consent" can engage in "private conduct" without government intervention - then why can't three, or four?
A reading of Polyamory in Wikipedia showed me that the law is still pretty tangled up, even though Lawrence v. Texas has resulted in some state legislators reviewing their laws on marriage. According to the Wikipedia article:
Bigamy is the act of marrying one person while already being married to another, and is legally prohibited in most countries where monogamy is the cultural norm. Some bigamy statutes are broad enough to potentially encompass polyamorous relationships involving cohabitation, even if none of the participants claim marriage to more than one partner. For instance, under Utah Code 76-7-101, "A person is guilty of bigamy when, knowing he has a husband or wife or knowing the other person has a husband or wife, the person purports to marry another person or cohabits with another person."
Having multiple non-marital partners, even if married to one, is legal in most U.S. jurisdictions; at most it constitutes grounds for divorce if the spouse is non-consenting, or feels that the interest in a further partner has destabilized the marriage. In jurisdictions where civil unions or registered partnerships are recognized, the same principle applies to divorce in those contexts. There are exceptions to this: in North Carolina, a spouse can sue a third party for causing "loss of affection" in or "criminal conversation" (adultery) with their spouse, and more than twenty states in the US have laws against adultery although they are infrequently enforced.
So, the law seems to be all over the place, and some conservatives believe that the ruling in Lawrence v. Texas opens the floodgates to polyamorous marriage. And there are several forms that a poly marriage can take, which further complicates any possible future changes to marriage law.
But as someone who bases my decisions on reason and rationality, I cannot see any ethical reason to condemn a relationship between "n" number of people, as long as everyone in that relationship gives their informed consent. I can see a legal reason to prohibit such a union - in that today's laws are not constructed to be fair to such a marriage. But I can see a way to counter that objection by learning from and perhaps adapting laws of incorporation to marriage. Why not? A couple is often legally treated as a union, a single entity; and so is a corporation.
Santorm smugly thinks he has found a counter to same sex marriage, but I think he is one of many who (unknowingly) are working for the legality of plural marriage.