What about a compromise? Maybe you and Seven can agree to not show any of your M/s elements publicly or in front of her, but still be allowed to have it in private.
Then, of course, she needs to let him know what needs she has that she feels are not being met by him. However, I fail to see how she determined that just a M/s relationship between you two is the factor in that, and not any kind of relationship. It sounds more like envy. I think it's a great idea to all go to therapy together.