This is a completely stupid argument for denying gays the right to wed. The state cannot sanction a marriage where the consumation of said marriage would violate the law.

For that reason, marriage between closely related people can constitutionally be denied because all states have laws that make incest a crime (and the laws banning incest are constitutional because of the far greater likelihood of brain damaged offspring). Likewise, "marriages" between a person and an animal are legitimately denied because the consumation of those marriages would violate state laws against animal abuse.

This line of argument was first proposed by a Republican congressman (whose face I remember although I can't recall his name), and I remember how stupid he sounded when he made the argument. Anyone with a high school education should be able to see right through the argument, but unfortunately opponents of gay marriage will grab at any justification they can find.