This isn't the way rational basis review works. The question isn't whether the legislature could have come up with a law that better suits the law's purpose. The question is whether the law in question is supported by a rational basis. Traditional marriage laws plainly satisfy that standard for the reasons I've set forth. The only way a court could legitimately strike down traditional marriage statutes within the current legal framework would be to create some sort of favored status for homosexuals. In that regard, activist judges have been all too willing to comply.
Moreover, even if one were to accept your incorrect characterization of rational basis review, your argument still fails. A legislature could rationally conclude that giving favored status to traditional marriage (over and above same-sex marriage) is more likely to encourage some people to enter into it and, thereby, have kids that they otherwise wouldn't have. This could occur in at least two ways. First, a legislature could rationally conclude that some portion of homosexuals prefer members of their own sex by choice (conscious or otherwise) and that, by offering them an option to be married to a member of their own sex, they are encouraging behavior that is unlikely to result in procreation. Second, a legislature could rationally conclude that expanding marriage beyond its traditional definition to include same-sex unions diminishes the societal uniqueness and importance of marriage, particularly insofar as one of the traditional purposes of marriage is for procreation, thereby deterring heterosexuals from entering into it. This would decrease both the likelihood of procreation and the likelihood that a stable environment awaits the children who are born.
I personally believe that same-sex unions are good policy. But they are not required by the constitution on anything that resembles a faithful interpretation of it.
