As I repeatedly answered in this thread, the invention of "homosexual marriage" is against Christian doctrine at a much deeper level than just legitimizing illegitimate lust. It is, in fact, the umpteenth rebirth of the never-enough-silenced heresy of Docetism - the first recorded heresy in Christian history: that is the idea that the physical nature of man has nothing to do with man's essence, that the whole business of physical life is nothing to do with the spiritual and with God. (Docetism began with a rejection of the idea that God could have wholly incarnated Himself, let alone died.) If two souls can be sacramentally joined in despite of their physical features, then the whole idea of body and soul as one, and of God becoming incarnate to die as a man, is completely destroyed. And you might understand that this is an attack on Christianity a bit more extreme than even the legalization of lust as such.