A couple commenters beat me to it, but the self-paradoically specious argument in defense of marriage discrimination djw makes fun of below was, less than ten years ago, good enough for the New York Court of Appeals:
Despite the advances of science, it remains true that the vast majority of children are born as a result of a sexual relationship between a man and a woman, and the Legislature could find that this will continue to be true. The Legislature could also find that such relationships are all too often casual or temporary. It could find that an important function of marriage is to create more stability and permanence in the relationships that cause children to be born. It thus could choose to offer an inducement—in the form of marriage and its attendant benefits—to opposite-sex couples who make a solemn, long-term commitment to each other.
The Legislature could find that this rationale for marriage does not apply with comparable force to same-sex couples. These couples can become parents by adoption, or by artificial insemination or other technological marvels, but they do not become parents as a result of accident or impulse.
Despite my joke in the title, the thing is that this silly “marriage laws are about creating more shotgun marriages” argument does sort of qualify under the traditional “rational basis” standard, which pretty much requires “any reason that can be stated in a reasonably coherent sentence” as opposed to “an arguably good reason.” This is why the traditional rational basis standard is inappropriate to a fundamental rights case like this, and why the allegedly “rational basis” reasoning of Romer wasn’t really. If the courts use a standard with any teeth this farcical justification collapses instantly as a basis for denying a class of people a fundamental right, and remember that the New York courts considered this one of the two best arguments.