Some points:
1 - The "slippery slope" issue is a real one that really bothers a lot of people. If we give up on our millennia-old traditions, what do we put in their place? Before, there was a standard that kept out of bounds a whole lot of things that bother a whole lot of people. It was like having a fence that no one had ever moved. The immovability of the fence was the guarantee that all these things wouldn't happen. In order to let gay marriage in we have to move the fence. But now we have two issues: gay marriage and moving the fence.
2 - Gay marriage has arisen as an issue I believe for two reasons: first, the government and various companies have guaranteed spousal benefits, and gay couples want in on that game; second, we have identified marriage with romance, and gay partners are romantic. As long as we say "we should get married to the one we love" we will have gay couples wanting to get married.
3 - "Love" in this case means romantic love. I do not believe that different definitions of the same word by themselves constitute a slippery slope. When I say "I love potatoes" I'm talking about a different state than when I say "I love my girlfriend", therefore they have different ramifications both socially and legally. As an example of the same type, most governments would not want to stop me if I said "I fix cars", but they would want to do so if I said "I fix elections".
4 - This is about homosexuality. People shouldn't expect others to feel confortable about putting legal sanction upon a behavior that they separately find disturbing or immoral. If "disturbing and immoral" define the location of the fence, then including one disturbing and immoral thing is the same kind of action as including any other - ergo, gay marriage and dog marriage are on the same slippery slope.
5 - The issue is still not so much "should I marry my dog" as much as it is "should the government grant me and my dog priviledges that we cannot otherwise get unless we are married." Gays get married all the time - in churches even. The question is whether or not those marriages should have legal standing or not. This is important because many legal benefits are extended to brothers as if they were married. E.g. my brother can visit me in the hospital when my best friend cannot. Certain insurance benefits cover children if they cover parents, etc. So I don't have to marry my brother in order to trigger these things to happen. To me, the real disturbing thing is that friends get nothing in our legal system. I could be living with my best friend and mortally afraid of my abusive parents, but who has more legal rights to me - the abusive parents.
And I still think as I have said before that we need to figure out why straight marriage is legal before we can figure out whether gay marriage should be.
