Beyond the obvious fact that homosexuals are unable to reproduce, this is an even more obvious reason why marriages should be reserved for heterosexuals. Let the homo crowd have civil unions or something else which doesn't redefine marriage in such a way as to render it meaningless.
Sexual intercourse is something we breeders occasionally do, it doesn't define who we are.
Part of the reason for unfaithfulness may be that homosexual behavior may not cause the same alleged bonding effects as heterosexual behavior, due to different equipment being used. Just a thought.