No, its not the word "lesbian." Its the implication that a) you have a need to know whether she's lesbian or not; and b) that her status as lesbian or otherwise is relevant to the real issue, which is her status as a target of harassment.

I get that you're not trying to be offensive. What I don't get the sense you understand is that you don't have an inherent right to know things like her sexual orientation, history, background, favorite color, or anything else she doesn't volunteer.