Well, my question is... unless you know for 100% sure... how do you that person is or isn't?
I grew up going to the bathroom with my mother and cousin. My cousin and I would sit on the sink or washer, depending on who was where and who got there first and talk to my mother.
I've only had problems one time with going to the bathroom with someone that I knew was a lesbian. It wasn't her choice of relationship that bothered me. It was the fact that she was hitting on me where we worked at. What made it all worse was her S/O just happened to work there too and was usually in the same room.
My frame of mind is alot different now that what it was 7-8 years ago. But still, it wouldn't bother me in the least. As long as they aren't coming after me and I'm not interested in them.
