Judge Walker in California actually made a text-book example of the militant-gay response to rational inquiry along these lines. He dismissed the rational basis of procreative interests and generational succession in a mother-father marriage by claiming that no one was required to prove they were capable of procreation to obtain a marriage license.
That was a logic fallacy.
It’s actually a handy rhetorical device - if you want to be dishonest. Instead of dealing with the point directly, you
argue the inverse of the point and hope no one remembers the basic logic components of geometry. (Whether a Statement is True or False has no bearing on whether its Inverse is True or False)
Judge Walker shot his own case in the foot and it will be made readily apparent at the Supreme Court. The laws
do prohibit opposite-sex couples from marrying on the basis of their inability to fulfill the procreative role of marriage being a matter of public record. In-breeding is a basis on which civil marriage is denied.
If we take away the reproductive component of marriage their is no longer a Public Interest in the sexual congress of the two partners. The State has no rational basis to treat such a union as any more special or important than a business partnership or some other manner of private contract.