As long as they are marrying for love then what difference does it make to anyone what gender they are? Now a man and a woman cynically marrying for 'convenience', money, to keep family happy, etc when they don't love each other? THAT diminishes marriage.
Ok then guys and dolls, Say two blokes got married and one of them slept with another person causing divorce why cant they call it aldultery but they can for man and woman relationships. If they want marriage to be equal for all types of couples why is adultery only for heterosexual couples.