The ins and outs of the slippery slope

Dahlia Lithwick explains how gay marriage can and cannot be argued against with the slippery slope argument. This is a good article and it explains how important the issue of consent informs the modern legal view of sex, an issue the right wing notably is trying to ignore and undermine at every turn. She also draws a distinction between gay marriage and polygamy:

But one can plausibly argue that there is a rational basis for states to ban polygamous and polyamorous marriages in which there has been historical evidence of an imbalance of power, coercion (particularly of young girls), and an enormous financial burden placed on the state. None of these arguments can be made against gay marriage.

It seems to me that polygamy is illegal for absolutely different reasons than gay marriage is. Certainly not because it is immoral by right wing standards; my guess is that if religious fundamentalists started agitating to bring it back because the Biblical patriarchs had it, they would get plenty of support from the right. Polygamy is illegal for the same reason that gay marriage should be legal, because the definition of legal marriage has changed from the ownership model, where a man owns his wife and therefore why shouldn't have as many as he can afford, to a partnership model, where both partners are equal so why should they be different sexes? I expect that is exactly why it will be shot down in court should polygamous men sue for the right to legally marry as many teenage girls as they would like.