Indefensible on constitutional grounds. That doesn't mean the Republican social conservative base understands this, of course, and will want to waste money on a kamikaze run.
Again, to gay rights activists. The DOMA is perfectly defensible on constitutional grounds. Nothing in the constitution confers rights to people due to sexual orientation, or so the argument goes.
But it doesn't, because there is no such thing as a legal concept of "same-sex" marriage. Nor is their a federal marriage license for which the federal government could set standards. What there is a federal law saying a marriage certificate issued by the state of Massachusetts makes one eligible for certain benefits, unless it is issued to a certain group of people.
This is not about whether a federal law defining marriage, and taking over the issuance of licenses is constitutional. Such a law almost certainly is. It is whether the federal government can outsource it to states, but then deny recogniztion to state recognized partnerships on a basis of guess work. Since again, MA does not report to the Federal government which marriages are same-sex and which aren't.
Again, what pray tell, is the legal difference between a Massachusetts Marriage license issued to Sam and Steve and Sam and Paula. Not what is the justification for treating them differently, which is the prerogative of the state of Massachusetts, but what makes them legally distinct?