Slippery Slope or Religious Freedom? Mutually Exclusive Arguments

I want to undertake a very brief thought exercise.

One of the most common arguments against same-sex marriage is that allowing same-sex marriage would be the first step down a slippery slope that would lead, inevitably, to polygamy. Now, I don’t want to get into a discussion about why that argument is wrong, why there is no slippery slope, or why polygamy is fundamentally different than same-sex marriage. I’ll save those discussions for another day. For that matter, I’ll also leave for another day any discussion of polygamy itself.

Instead, I’d just like to compare that slippery slope argument with the new objection to same-sex marriage that has been getting lots of attention, lately: the infringement of the “religious freedom” of those who oppose homosexuality.

Do you see the problem?

On one hand, people are saying we can’t allow same-sex marriage because that will lead to polygamy. But at the same time, they’re saying, we can’t allow same-sex marriage because it infringes on their religious freedom. Yet doesn’t a prohibition on polygamy directly infringe upon the religious freedom of fundamentalist Mormons and Muslims (and any other groups with religious texts allowing or encouraging polygamy)? And isn’t that infringement upon the religious freedom of polygamists much more direct —and with a much greater impact — than the “impact” to a religious person of allowing a same-sex couple to marry?

It seems to my that same-sex marriage opponents have destroyed at least one of their own arguments here. Either they don’t really care about the so-called slippery slope to polygamy because they recognize that efforts to stop polygamy are infringements upon religious freedom or religious freedom isn’t really that meaningful if it involves a religious viewpoint with which they disagree (or someone else’s religious freedom).

If I’m missing something, let me know, but it appears that the argument based on polygamy and the argument based on religious freedom are mutually exclusive.