Well, lessee now.
Under the "Full Faith and Credit" provision of the Federal Constitution, if I've got a driver's license from Kentucky, I can legally drive in Massachusetts.
If I've got a driver's license from Vermont, I can legally drive in Massachusetts.
If I've got a Concealed Carry permit from Kentucky, I CAN'T legally carry concealed in Massachusetts.
If Massachusetts refuses to recognize self-defense provisions of other states, even though they are Constitutionally obligated to, then the rest of the states have no obligation to recognize said "marriages" outside of MA.
Following your logic, the SCOTUS should never have ruled that the Texas sodomy law was unconstitutional, but they did.
The next logical step in the gay marriage ladder is for a challenge in one of the 37 states that have a DOMA on the books (MA did not). With the decision of the SCOTUS overturning the Texas sodomy law, it would certainly seem that there would be precedent for them to overturn the DOMA in the 37 states that have them.
When marriage is no longer narrowly defined, then it can mean anything to anyone. That is what Scalia warned about.