Slippery Slopes and Gay Marriage

I was just reading some commentary on the legalization of same-sex marriage in Massachusetts, Connecticut, Iowa, Vermont, Maine, and New Hampshire. And Steve Capman notes

Opponents of same-sex marriage reject it on religious and moral grounds but also on practical ones. If we let homosexuals marry, they believe, a parade of horribles will follow—the weakening of marriage as an institution, children at increased risk of broken homes, the eventual legalization of polygamy, and who knows what all.

In other words, some opponents to same-sex marriage make the slippery slope argument, that it will lead to progressively more indecent and immoral forms of marriage and more divorce.

Admittedly, I have no objection to slippery slope arguments in general. They're certainly not logically rigorous, but they are meant to be predictive and are based on an accurate understanding of human psychology. People become accustomed to something and then can become more willing to accept even greater extremes. In retrospect it seems reasonable that when the the US Supreme Court in 1937 changed their opposition to Roosevelt's New Deal after Roosevelt's court-packing threat, that this has led to a continual erosion of the perceived constitutional limits on the executive branch and has been partially responsible for the semi-regal office of president we have today. One could've objected in 1937 that permitting a little bit of extra power to the president in a time of emergency, could lead to the president ultimately having quite excessive and unchecked power. This argument would be correct.

The most common mistake of the slippery slope argument is to assume some sort of inevitability to it. It may seem inevitable in retrospect, but this is misleading.

In the case of same-sex marriage, though, the mistake is in misidentifying the slippery slope. Acceptance of same-sex marriage is the result of greater tolerance of homosexuality. If we have more same-sex marriage then it could lead to a slippery slope of greater tolerance for people being open about their homosexuality, for more institutions catering to homosexuals, for openly homosexual public figures, like politicians, business leaders, celebrities and so on (which I see no problem with), but it doesn't seem like a slippery slope towards erosion of the institution of marriage. Over time people will learn more to accept the equal sanctity of both heterosexual and homosexual marriage. Same-sex couples are entering into marriage because they want to affirm the type of life-long commitment that heterosexual couples already find in marriage. Increased incidence of divorce seems to be a separate phenomenon, probably more connected to increasing independence of women, a greater emphasis on individualism and autonomy.

That same-sex marriage would lead polygamy is absurd, since that would have to come as a result of greater tolerance of polygamy or polyamory or religions that promote polygamy (like Mormonism). In fact, it seems like our increasing emphasis on women's rights would lead us away from polygamy (though possible towards acceptance of polyandry). And acceptance of incest or bestiality or pedophilia? These are also completely unrelated to acceptance of homosexuality.

If someone opposes same-sex marriage on moral or religious grounds, it's difficult to argue with them, but if they want to make the argument on practical grounds, I think that argument falls apart.