I was married out of the U.S., so I'm not sure what the requirements are, but I would assume that the basics would be some form of legal identification. There might be a clerk who would accept floating above the ground as proof of identity, but civil servants are known to be rule sticklers, so they might run into someone who wouldn't. Then, again, since they can fly around the world fairly quickly and easily, I'm sure they can find a country and/or church where people wouldn't be so picky about the rules.
Is it necessary that Superman and Ultra Woman get married as opposed to Lois and Clark? If LnC are already married legally, and nobody knows about the destruction of Krypton, they could always say that their union had already been sanctified (is that the right word?) "elsewhere".
