I'm curious why you think this is the case. I guess it probably depends enormously on the type of therapy you're doing. I have the kind of therapist who will analyze the occasional dream, and she was fine with me talking about what I imagined her private romantic/sexual life is like. Same when I was going through fertility stuff -- I assumed (rather angrily and perhaps completely incorrectly) that having kids came easily for her because she's straight. She shares almost nothing in this domain, but discussing what I think and feel about anything that seems relevant to me is often fruitful.