If they say that banning gay marriage in unconstitutional, then they have to rule that banning any type of consensual marriage in unconstitutional.
Then again, when they repealed the sodomy laws, they failed to repeal all the laws against incest and bestiality. No consistency in the courts anymore.
The legality of incest and bestiality weren't questions before the Court in
Lawrence v. Texas, but Justice Scalia made the point in his dissent that if the Court did not want to uphold laws based on morality that laws against gay marriage, incest, bestiality, polygamy, etc. would not stand. His
dissent is worth the read.