Please ask yourself the following questions before asking her:

Why do you want to ask her? I know most people would love to have lunch with their T, but I am wondering if this isn't a way to test her boundaries.

How would you feel if she says "yes, lets go have lunch!", will you be concerned that her boundaries are "too loose"? What about if she says "no, it's outside the scope of our professional relationship". Will you be angry that she crossed all sorts of boundaries before and now she is "pulling back"?

Personally, I think testing a therapist's boundaries is a no-win situation (unless the therapist has been extremely consistent and firm with their boundaries). If they let a patient cross the boundary, the patient will continue to push.

Moreover, I sometimes think that there is some ambivalence about therapists who let patient cross boundaries - while the patient is originally thrilled and made to feel "special", there is also the growing suspicious that the therapist is not being professional and using the patient to gratify their needs.

Given what you have said about your T in the past, I think it's a bad idea. But ultimately it is your decision and you will do as you intend. If I remember correctly (my apologies if I am incorrect), you were struggling about telling your T that you are bi and might have some feelings for her? I would explore that first.

Take care,