Spousal rights regarding testimony in a trial?
I'm glad you've brought this up, because it's a huge issue -- one that pretty clearly implicates the Due Process and Equal Protection Clauses of the Fourteenth Amendment.
There are two distinct marital rights as regards testimony; one is the privilege protecting private spousal communications, the other is the right to refuse to testify against one's spouse. Right now, so far as I know, same-sex couples do not enjoy either of these protections anywhere in the U.S. except (I assume) Massachusetts and (now, I further assume) California.