Ah, but it is exaclty here that the problem lies, as there is something than makes B distinct from A: Animals are not human.

Even if we assume that animals have minds, their minds are nothing even close to the sophisication of humans.
Even if we assume that animals are capable of love, their love is nothing even close to the sophistication of humans.
Even if we assume that animals have souls, their souls are nothing even close to the sophistication of humans.

One of the strongest arguments that we have often brought up is that there is some point of spiritual and mental maturity before the onset of which sexual interaction is immoral because that person cannot fully comprehend the meaning of the acts. The most advanced animals on this planet (aside from humans for those jokers out there) can achieve at best the comprehension of a 3 y/o. No-one here has ever (to my knowledge) asserted honestly that a 3 y/o can fully consent to sex, and certainly not to the binding of marriage. Then we must be able to say that full sentience is necessary for a valid marriage.

And I do not believe any of those aforementioned assumptions, and there is pretty good science supporting at least the first two. While I'm not going to tell a 10 y/o little boy whose dog just died that dogs don't have souls, I have strong doubts that the spiritual energy that makes up a dog's spirit remains coherant after death as does a human's.

The "slippery slope" argument can only be applied (and only loosely at that) if B is functionally identical to A, and these things are clear (to me at least) and fundamental functional differences in the basic nature of animals vs. humans in the relevant ways.

Thus, further steps must be made to move from the argument for A to an argument for B.

I want to touch on the other common assertion with the slippery slope argument: that of polygamy. As Godspell alluded to, the Bible was not only permissive of the taking of multiple wives, it has several laws about just how it is to be done. However, like slavery, society (most of it at least) has learned (from the law writ upon our hearts?) that by and large polygamy is wrong and degrading of women. I must concede, though, that I think I might be swayed to be accepting of some limited form of polygamy. There are some good starting points, and I can see the beginings of some solid paths to follow there. I'm not saying I'm already there, just that it might be the case, and with some careful boundaries could maybe still be valid in today's society. Maybe. Fodder for another discussion. In like 40 years.

Love in Christ,
~CSL