The March of Progress
The current same-sex marriage fad is easily explained.
From the beginning of our species, the female has performed the most important functions-childbearing and homemaking. All she asked of the male was that he go to work and provide for the family.
Then the Women's Liberation Movement insisted that women don't need men even for that purpose, with artificial insemination and the Welfare State to support them.
So, now that women no longer require husbands, men may as well marry other men.