Slippery Slopes and Parallel Rhetoric

Philocrites is worried that pushing for polyamory to be recognized will give aid and comfort to people who oppose same-sex marriage due to fears of a slippery slope. One would think that he would therefore be careful to present the two issues as clearly different cases to which different reasons apply. So it was with some befuddlement that I took his recommendation to read this post by LT laying out the anti-polyamory case, because LT uses almost the exact set of arguments that social conservatives use against same-sex marriage (edited to add specific quotes from LT):

It's contrary to the way we've done things for hundreds of years -- check ("Monogamy has been the norm in this culture for a long time.")

The burden of proof is on innovators to show they won't disrupt society -- check ("I believe that those who propose changing our collective position on two-person marriage bear the burden of proof that such a change is warranted, necessary, and socially responsible.")

It will harm kids in some unspecified way -- check (Polyamory advocates must "demonstrate that the widespread and eventual legal recognition of multi-partnered relationships ... will increase the stability of families for the benefit of children, women and the social order.")

It's really just about overprivileged people indulging their selfishness and avoiding responsibility -- check ("There has been, also, a bohemian rebellion against monogamy among the more privileged sets for quite a while..." "Our experiments with "open marriage" in the 60's-70's proved to be not prophetic of a liberated future, but an exercise in self-indulgence.")

If everybody did that, society would fall apart -- check ("One of my tests is a question that my mother used to ask me when she objected to some aspect of my behavior, "What if everybody did that?"")

It's a choice, so I don't have to respect it -- check ("They have adopted the stance that the potential disapproval of their fellow congregants is a prejudice against them, similar to homophobia, or racism. But there is no convincing evidence offered that being in a multi-partnered relationship is anything other than a choice that they have made.")

The only ones I see missing are "it's against God's law" (an argument the Judeo-Christian tradition cannot support) and "it's physically unhealthy."

While I'm editing, I should clarify as well that my point is about the rhetorical relationship between these arguments and slippery slope concerns. Whether they're valid arguments is a different post.