Until very recently, same-sex marriage was unknown in human history, and it is opposed today by many progressive leaders, like Obama and Clinton. Can this be explained only by irrational prejudice or religious zeal? No. Only unions between men and women are capable of producing offspring, and every civilization has recognized that responsible procreation is critical to its survival. After the desire for self-preservation, sexual passion is probably the most powerful drive in human nature. Heterosexual intercourse naturally produces children, sometimes unintentionally and only after nine months.Judge Walker's ruling for same-sex marriage is based on the premise that no rational person could believe this argument.
Without marriage, men often would be uncertain about paternity or indifferent to it. If left unchecked, many men would have little incentive to invest in the rearing of their offspring, and the ensuing irresponsibility would have made the development of civilization impossible.
The fundamental purpose of marriage is to encourage biological parents, especially fathers, to take responsibility for their children. Because this institution responds to a phenomenon uniquely created by heterosexual intercourse, the meaning of marriage has always been inseparable from the problem it addresses.
Homosexual relationships (and lots of others as well), have nothing to do with the purpose of marriage, which is why marriage does not extend to them. Constitutional doctrine requires only one conceivable rational reason for a law, and the traditional definition of marriage easily meets that test.
There is indeed a trend to eliminate the above purpose of marriage. The idea is that with DNA testing, massive national databases, and draconian family court laws, fathers can be forced to take financial responsibility for their children. Feminists argue that no non-financial responsibility is needed, as single moms can raise kids as long as they get enough welfare, food stamps, medical care, and forced child support payments from the biological dad.
It is an open question whether a civilization can be built on such an idea. That appears to be where we are headed, so I guess that we will find out. The evidence so far is that the kids reared by single moms do worse by every available measure.