It is more complicated than that. Now that ye did have sex once you can play it down as just being sex. Worst case scenario" one for the road".
But the thing is, she is clear your M is over and she is moving out. She wants to do fun stuff with you and use you for sex, all while being clear it is over.
I can think of worse ways of being used, but that is what she is doing.And you are OK with it. Maybe you can reconnect with her by sharing good moments together. Idk. And for sure it will be more enjoyable until she leaves. But you are giving her everything she wants so she does not have to reconsider.
We all have a choice in what path we choose and only one person walks in our shoes. I am not sure that being her sex friend is the best route for you to choose to save your M. I am no expert so maybe a coach could be better..
If she does leave as she intends, how do you want to look back on this time?
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together