Yes, I am in favor of same-sex marriage.
Civil secular marriage exists for the purpose of creating a legally recognized next-of-kin family relationship between two consenting adults who would otherwise be legal strangers to one another, to make it possible for them to take care of each other -- leave property to each other, make medical and financial decisions for each other when necessary, sponsor each other for a green card if one partner is not a U.S. citizen, put each other on their company health insurance policy, etc, etc etc. Its a good thing for society to encourage people to take care of each other, that way the government doesn't have to.
Family members -- parent/child, siblings -- are ALREADY legally recognized next-of-kin and can ALREADY take care of each other in the ways described above, so there is no need for them to get a marriage license. What they do or don't do in the privacy of their home, behind closed doors, I don't care to know.
Marrying animals????? In case I didn't make it clear earlier -- marriage is a LEGAL CONTRACT. Your dog isn't legally competent to give consent to a legal contract or to be held accountable for living up to his/her end of the deal. And if you genuinely want your horse, or your cat, to make financial or medical decisions for you in case you're ever incapacitated, you need to have your head examined.
And how does one enter into a legal contract with oneself? That's just idiotic.
Same-sex marriage simply does away with the one-man one-woman requirement and opens it up to any TWO (not one, not three or more) consenting adults (NOT children, not dogs, cats, goats or horses), who are not already married to other people, and who are not already related to each other within a certain degree of consanguinity. Polygamy, OTOH, fundamentally changes the entire nature of the contract by introducing a third (or fourth or fifth) person into the relationship. Whole new laws concerning family support, child care, divorce, parental rights, etc etc, would have to be worked out. I'm not saying it couldn't be done, but it would be a whole new ball game. OTOH, I haven't seen any great public outcry in favor of polygamy, have you? I think if enough people want the option of polygamous marriage, they should organize a movement, start getting signatures on petitions and start making their case to the American people for why the laws should be changed to allow it. I'll listen.