My wife and I have gone for a couples message a few times. We talked about it for a long time in our "dirty talk" during sex. But as you noted things said during that time to not always translate to what folks wish to do outside of play time.
In the end I think what finally convinced her to go was one, the hot tub, where we went. As it offered a chance to relax just the two of us. The MP asked if we wanted a little time first to relax before the massage.
The other thing was it would be a massage and only would go further if she was comfortable with it having a happy ending. The girl who gave the massage was great at her work and helped her relax and in the end it was a very good time.