Actually, this is more of a resolution than a true law. It just means that an interpretation of the Constitution that says one state has to honor another state's same sex unions is invalid.
Considering that interpretation is wrong, then so is the resolution/law or whatever you want to call it.
Its wrong because of the Comity Clause:
"Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State."
Interestingly this never covered civil unions (i.e. the type that were administered in VT), but does cover same-sex marriage.
Though this isn't uconstitutional under the 10th amendment (more like the 2nd amendment), I'm going to add it as well:
Most of the gun laws (though there are a few that I can slip into Constitutionality)
