Marriage is a four-way covenant between a man, a woman, their culture and their children (who have the right to have parents of both genders). Through the ages, however, "marriage" devolved into:
• Princes marrying princesses to complete political alliances;
• Men buying wives;
• Men requiring wives with dowries;
• Men treating women as property;
• People of either gender marrying multiple spouses;
• The big house for the wife; the little house for the mistress;
• Men inflicting culturally accepted violence against women;
• No-fault, quickie divorces.
It's fascinating that when heterosexual culture corrupted the marriage institution, proponents called it "pragmatic" or "progressive." But when homosexuals lobby for same-gender marriage, family-values advocates call it "sin."
There is enough sin in marriage concepts to go around.
Through time, heterosexuals have done the most damage to the marriage institution. Changing gender requirements would be just another devolution.