POLYGAMY=SLAVERY=SAME-SEX MARRIAGE?

My admittedly limited brain power is overloaded by this post by Ken Masugi:

As Americans struggle with the issue of same-sex marriage, we should recall the 1856 platform of the Republican Party, which denounced those "twin relics of barbarism-- polygamy, and slavery." In light of Lawrence v. Texas, it is perfectly understandable that someone file suit on behalf of polygamy. . . . Will the current Republicans, and all other Americans who prize freedom, come to see same-sex marriage in the same light as the earlier Republicans saw polygamy and slavery? It's apparent that they understood the Constitution in light of the Declaration of Independence, not some fabrication of constitutional text. (links in original)

Seems like there are a lot of claims here:

• The Republican attacks on slavery and Mormons were based in the same principles.


• These principles were informed by a constitutional understanding: looking at the Constitution "in light of the Declaration."


• Same-sex marriage is sufficiently similar to slavery and polygamy to enjoy the same theoretical cover.


• If Republicans care about freedom (and the Declaration of Independence), they should be against same-sex marriage.

No support is offered for these claims (it's a short post, so perhaps I shouldn't be too picky). The only part of the post that makes sense to me is the claim that a constitutional understanding informed by the Declaration would be hostile toward slavery. It might also be hostile toward the militant, hierarchical, separatist Mormon community out west as a political phenomenon in the 1850s (although I'll admit that I don't know a lot about the history of the Mormon community). But polygamy as such? I really do not get this.

Any help would be appreciated. I'm especially looking for something that will help to make this post into a coherent whole rather than a seemingly random collection of sentences.