I think where the difficulty in comming to an appropriate "solution" to gay marriage is the fact that there is (at least) two distinct definitions of marriage which support opposite positions on the issue.
The "Traditional" definition of marriage more or less relates to a man and a woman entering into a partnership in order to raise children with legitimate paternity. This is the predominant view of marriage throughout the world, and has been the predominant view of marriage throughout history, and a relationship that can not generate children does not make sense under this definition ... A man declaring he is menstrating makes as much sense as two men declaring they're married under this definition.
The "Romantic" definition of marriage really started to exist as literacy increased, and then exploded as Hollywood brought movies into everyone's life. Essentially, under this definition two people enter into marriage as the ultimate symbol of their romantic love for eachother. Under this definition any two people entering into marriage makes sense as long as they love eachother and sexual preference doesn't matter.
The western world has moved more and more towards the "Romantic" definition of marriage for a couple of generations which has its good sides (a much more loving and romantic relationship) and its bad sides (far greater divorce rates). At the moment people who hold each definition are (roughly) equal in size and until something changes there will be people on both sides who try to enact legislation to support their view of marriage.
