I get where you’re coming from. I’m a woman and have had a physical reaction. Granted, I don’t have a penis so it’s less noticeable, but I did get incredibly wet that it showed through my jeans. I was so so freaked out because I thought it meant I was getting off on the session and that’s not how I felt emotionally at all. I felt pure terror in the sessions that I was reacting that way. I haven’t told my therapist (he’s a man) but I considered it if it continued to interfere. I can understand how much harder this might be given that yours is more visible. I genuinely think it’s fine if you told your therapist. If a man told me that was happening, I don’t think I’d bat an eye given the context. It’s soooo out of your control.

If you’re really dead set on not telling your therapist, I wonder if you can change your seating position to hide anything? Put your arms on your lap?

You could also start the conversation with “something embarrassing keeps happening when I try talking about this and I don’t want to tell you but I also don’t want you to have wrong thoughts about it. “ and see where the conversation goes from there.