As a Catholic, I believe that marriage is a sacrament shared by two people who want to be committed to each other, who will love each other (because to continue to love someone is, in fact, sometimes a decision), and perhaps raise a family together, if they are able and so inclined.
However, I also believe that marriage is a civil right. The definion of "family" does no longer need to be defined as a man, a woman, and whatever children they may have. I believe children to be better served by two parents, of either gender, who have the emotional and financial maturity to care for their children and provide them a home with two parents who can provide a good upbringing, complete with morals, which have little, if anything, to do with sexuality: "It is the quality of the parent/child relationship and not the parent's sexual orientation that has an effect on a child's development."
By the way, the vows the first lesbian couple married in New York are the same vows Ed and I exchanged at our ceremony, held in a Catholic church.