I'm sure any of the cruise lines that perform marriage, commitment or renewal of vows ceremonies would be more than happy to perform one for you. However, as very few states in the US (I believe only Vermont and Hawaii??) legally recognize Gay & Lesbian marriages, the ceremonies would not be legally binding once you arrived back in the states.
I am going to touch base with the folks at some of the major cruise lines and get you some definitive answers on the ceremonies, but I'm pretty sure none of them would be "legally binding" unless you live in a state that recognizes such unions.
Consumer Affairs/Gay & Lesbian Editor