Same-sex "Marriage" and Polyamorous "Marriage"

In an earlier post, I taked about the Newsweek article on polyamory. I believe that those who are for same-sex "marriage" today will be in favor of polyamory tomorrow, even if they don't know it yet. Why? Because there are no cogent arguments in favor of SSM that don't also justify legalizing polyamorous "marriage" (hereafter PM and SSM).

Here is a thesis for debate:

"You can't justify legalizing SSM and not legalizing PM."

This is what I think. But I'd like to hear arguments on the other side. If you want to take a crack at proving the above proposition false, all you have to do is e-mail me your argument (500-1000 words) and if it is any good at all I'll make it a guest post. [ccarter@tyndale.ca] Then I'll reply to it and give you a chance to reply to my reply. (Since it is my blog, inevitably I'll get the last word!)

One rule: you can't dismiss my thesis with a reference to "slippery slopes." That is a red herring. My contention is not that being for SSM leads to being in favor of PM, it is rather that anyone in favor of legalizing SSM is already in favor of legalizing PM whether he or she realizes it or not. There is no slope. To be in favor of one is already to have adopted the necessary arguments for justifying the other. If you are in favor of SSM, you already believe in PM, even if you have not thought your way through to your conclusion. In order to avoid supporting PM, you would need to change your arguments for SSM.

Is anyone up for it? Any takers? Keep it clean, as they say on the sports pages blogs, and we can have a real debate.