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There must be thirty letters stating that wearing jeans and a t-shirt and not shaving one's legs doesn't make someone a lesbian and shouldn't make people think they are lesbians. Of course not. The only problem is, that's a big straw man.
Human beings are social creatures and the way we dress sends signals about a million things. What we find comfortable is certainly one of them, but so is social class, religion, politics, and, yes Virginia, sexual orientation. If all your life people think you are a lesbian it is because you are sending out signals that say "Hey world, I'm a lesbian." Now there are a number of possibilities here. Maybe you are socially inept or come from another culture and don't know the signals of this time and place. That letter would have read "I don't know why everybody thinks I'm a lesbian." Clearly this LW knows why and is unsurprised when they do. Another possibility is that you just don't give a damn what people think and you want to wear what is most comfortable. Again, I think this is unlikely for reasons that I explain below.
Another possibility is that the LW knows she sends out "I'm a lesbian" signals and does it on purpose. This is Cary's view and I think he's right. It is clear that the LW identifies with and defines herself in relation to queer culture and community. For example she knows she is being harassed and yet her concern is about coming off as homophobic; she spends the entire third paragraph talking about her relationship with gays and lesbians and her own meditations on her relative attraction to women and men (which I'm sure we can agree has nothing to do with her harassment); and finally she signs her letter "A GBLTQ Ally" (rather than say "harassed employee" or "tired of people gossiping about my sexuality" or whatever else). The LW learly knows what she is doing and she certainly gives a damn.
Cary's advice is spot on, both about the harassment and about the issues she has with sending out "I'm a lesbian signals". He doesn't tell her she's wrong or that she should stop. He suggests that she think about why she chooses to send out those signals. Maybe he is right, maybe it's a trickster or culture jamming/gender f...ing thing. More power to her if that's the case. Maybe it's something else entirely.
Cary is counseling a little introspection, a very good thing for all of us. He is not saying that jeans and a t-shirt make you look like a lesbian.