The law, precedent, and tradition all agree on what individuals are eligible for marriage or any legal equivalent. Those who want to change it have the greater obligation to state persuasively why the change is necessary.
But naturally it is also fair to ask why things are the way they are. That is very straightforward. I repeat my statistics:
Number of people on this planet born of heterosexual unions: billions and billions
Number of people born of homosexual unions: zero.
The idea that we are somehow beyond the need for the state to involve itself in the regulation of procreation is laughable. Marriage is the institution around which this regulation centers.
To stop excluding gay couples from the legal structures of marriage is tantamount to stating that, as a matter of principle, procreation is irrelevant to marriage.
I'm not swayed an inch by the tradition argument. Many traditions have been lost because they were contrary to human rights, fell out of favor, or just didn't fit anymore. Of course my favorite is that if the women doesn't prduce a child we can try another woman. I miss the old days. Besides my wife has some beautiful sisters.