I think the thing that would benefit you the most would be to ask your t why she is encouraging it, exactly. And also to tell her that you’ve had fantasies of doing such a thing.

I don’t think it’s clear-cut at all that anything unethical is happening, but what really matters here is the t’s level of expertise with your particular kind of issues, and her motivation for encouraging the behavior. Sexuality and masturbation aren’t, after all, just adult things but something that lasts through the entire lifespan. Many children touch their genitals openly in front of adults, and many parents react strongly and angrily, instilling the child with a strong sense of shame.

I don’t know enough about these disorders or how they’re treated to say yes or no, but I can suggest finding out more about what’s behind her thought and paying close attention to whether she struggles to answer it. If she’s doing it purely for your benefit and not in any way for her own gratification, she should have an easy enough answer.