Now you're being silly. I'm not posting again on this thread until someone says something new and sensible.
OK, how about this, then:
The primary purpose of marriage is to protect the ownership of property and its transmission to successive generations.
By encouraging the ready availability of sexual activity between a specific couple there is the greater likelihood that children are the product of the couple concerned. Hence family wealth is retained within the family. Religion became involved when religious leaders realised that the personal attachment between two people could be controlled by inducing guilt and shame into the relationship.
Modern marriage (due to the influence of religion) is a rather confused state with sex being subject to all kinds of moral controls.
One of the practical effects of marriage is that property can pass between spouses without control or taxation. Same sex marriage is a device which enables two men or two women who love each other the same property rights as conventional married couples. It also provides the opportunity for such couples to arrange for any children born to or conceived by one or other of the couple to become full members of a family and enjoy the rights possessed by children from conventional marriages.
And anyway, what people choose to do with the contents of their underwear is no concern of anyone else.