something?
Or do I know she's a lesbian because I know the person we'll enough that she's let me know that about herself?
Or do I know she's a lesbian because she's an outspoken celeb like Ellen?
In the first case, if her shirt DIDN'T declare her sexual preference, I'd just think it odd that a woman was in the men's room.
In the first case, if her shirt DID, I'd think it a publicity stunt for a cause, and consider it an odd place to make a stand (or sit, rather).
In the second case, since I know the person, I'd probably say hello, say it's nice to see her again, and ask her if one of us went into the other's restroom by mistake.
In the third case, I'd probably ask if I could take a selfie with her, so I could post something silly on social media that said something like, "You wouldn't BELIEVE what celebrity I met in the restroom, of all places!"