Well, I don’t think they are prohibited from testifying (it certainly doesn’t work like that in divorce court), they just can’t be compelled to testify against their spouse. If they want to testify, they are still free to do so.
As for the gay marriage issue, I believe that, according to DOMA, a state doesn’t have to honor gay marriages performed in other state, if they contravene the first state’s laws. So, I think that would probably apply to the ramifications of testifying in court also.
Again, I'm not a lawyer, so you may be right. But since that whole thing is dynamic, what will "settled law" in it be? I hate to think of the ways it could be twisted.
Thanks ---