When asked by a college student why he opposed gay marriage, presidential primary candidate Rick Santorum responded, "So, everybody has the right to be happy? ... So, if you're not happy unless you're married to five other people, is that OK?" suggesting gay marriage is comparable to polygamy. The slippery slope argument that gay marriage will lead to polygamy and incest is often used to oppose same-sex marriage.
It is especially ironic that Santorum would use polygamy as an example of the erosion of traditional marriage when historically, and biblically, polygamy was a perfectly acceptable traditional form of marriage. When marriage was redefined from "one man and many women" to "one man and one woman," was there a public outcry like we are seeing today in the same-sex marriage debate? Perhaps marital erosion began when society switched from polygamy to monogamy?
Opponents of gay marriage argue that same-sex marriage undermines traditional marriage because the purpose of marriage is to bear children. In that case, the traditional marital purpose of bearing children is more likely to be fulfilled in a polygamous marriage than a monogamous heterosexual marriage. Having multiple spouses increases the probability of childbearing. Polygamy seems to advance the goal of procreation far more than monogamous, heterosexual marriage.
The claim that homosexuality is unnatural is also an argument against same sex-marriage and, incidentally, an argument one could use to support polygamy in place of monogamy. When society switched to monogamy, I imagine people argued that it is unnatural for a man to be with only one woman, and that therefore, redefining marriage from polygamy to monogamy upsets the natural order of things. After all, man has been with many women since the beginning of time. Why tamper with the natural order?
The previous examples demonstrate how various social arguments against same-sex marriage also support polygamy in place of opposite-sex heterosexual marriage. The irony is striking.
Anyway, to further answer Santorum's question about distinguishing polygamy from gay marriage, here goes.
The arguments against polygamy don't stem from Judeo-Christian-Muslim values against same-sex marriage (values that historically permit polygamy!) but rather from the provable societal dangers associated with polygamy.
In 2008 the California Supreme Court distinguished polygamy from the right to same-sex marriage by explaining that polygamy is "inimical to the mutually supportive and healthy family relationships promoted by the constitutional right to marry." Polygamist leaders like Warren Jeffs, who last year was convicted of multiple sexual assaults and incest-related felony counts, illustrate how polygamy is inherently conducive to power imbalances, sexual subjugation, and other abuses that do not inherently exist in the case of same-sex marriage.
There isn't a shred of modern sociological evidence to support the claim that gay marriage is harmful to society, whereas there is a plethora of historical and contemporary evidence to illustrate the dangers associated with polygamy. One could even argue that there is less of a power imbalance in same-sex couples compared with opposite-sex couples, because both spouses are of the same sex. With opposite-sex couples, there is arguably a greater power imbalance because men are generally physically stronger than women. The bottom line is that the rate of domestic violence in both gay and straight marriage is basically the same. Aside from gender, the unions are exactly the same.
Every circumstance needs to be judged on its own merits. When looking at incest, for instance, it is quite clear that permitting consanguineous relationships will lead to power imbalances, psychological damage, sexual abuse, and a high rate of genetic diseases. Again, the basis for society's objection is not a religious one based on "family values" but one based on provable harm to society. The same cannot be said of two same-sex consenting adults getting married. Where is the evidence that children raised by gay parents are harmed? Where is the evidence that gay marriage will lead to the end of civilization? Show me one peer-reviewed, modern, mainstream study demonstrating the inherent dangers of gay marriage. You will not find it.
It is of course also ludicrous to claim that gay marriage, or a homosexual relationship, between two consenting adults has any connection whatsoever to pedophilia. Santorum has suggested this connection, too. Minors do not have the capacity to consent to sexual contact with adults, whether in the heterosexual or homosexual context. What about bestiality? Animals and humans are different species. You can't compare human-to-human relationships with human-to-animal relationships.
Finally, as an observant Jew and a heterosexual, married male, I see no conflict with supporting the legal right to gay marriage, even if I don't religiously support the union itself. According to the literal dictates of biblical law, atheism, homosexual intercourse, lobster, pork, and interfaith marriage between a gentile and a Jewish person are also forbidden, but I would not oppose a person's right to engage in any of those activities.
For fellow observant Jews claiming that Sheva mitzvot B'nei Noach laws demand that Orthodox Jews impose a biblical view of marriage on society, it is noteworthy that many rishonim (leading rabbis who lived during the 11th to 15th centuries, approximately), including the Ravad, Nachmanides, and Tosafot, rule that there is no obligation to impose Noachide laws on society.
At the end of the day, Jews and non-Jews should want to follow the dictates of their religion out of their own volition, not through coercive laws abridging people's right to marry.