As long as the person was in there simply doing his/her business it wouldn't bother me a bit. I've been out with other CDs and in the bathroom with them, and it's really no big deal to me. To be honest, I don't think the public is out there trying to identify CDs, and if you are clearly trying to blend in with the other women, they might not even notice (other than a passing thought that the woman washing her hands looked kind of like a man).
I think it would be extremely unfair to expect a fully presenting CDer to use the men's restroom, not to mention very dangerous for him.