Maryland Senator Barbara Mikulski has announced she will co-sponsor the bill to repeal the Defense of Marriage Act. I could be wrong on this, but with Maryland gearing up for a state-wide effort to gain marriage equality, she hasn't endorsed marriage in her own state. So, technically, she's one of those politicians who's not for marriage, just anti-DOMA.
Senator Mikulski has long been rumored to be a closeted lesbian. When people say that Tammy Baldwin would be the "first out lesbian in the Senate" if she wins her race, they're usually referring to Mikulski.
(Hat tip to JMG for the Twitter screenshot)