No, I'm convinced this one is worthy on its own, not specifically because of the 'gay' part, but the 'marriage' part. If we allow any state to define marriage around terms other than procreation, I guarantee you polygamy will be the next issue - with much more justification than gay marriage, since it's legal in many places, has a long tradition, and is arguably a religious practice.
#1) marriage is not currently defined around procreation--infertile couples are permitted to marry, and married couples can choose to remain childless.
#2) While gays cannot procreate through gay sex, they are still capable of making children through heterosexual sex and can adopt children--there are gay couples who raise children.
#3) Polygamy is not unsuited for procreation; in fact, polygamists tend to have big families.