It's a religious belief based in biological reality. Male-female pairing is observed throughout the animal kingdom, with good reason: it is necessary for species to survive. A "marriage" between two women or two men cannot exist, because it is not the biological mechanism to ensure propagation of the species. While they can be together for whatever reason they want, their being together cannot result in natural children, and has no biological purpose--so is not marriage.
Get right to it. When heterosexuals do what they do to make more heterosexuals it is called “love”, or “intimacy” or a number of other euphemisms. When homosexuals do what they do to make more homosexuals it is called pedophilia...