Here's the thing about boundaries. You need to know now, what you will not tolerate from other people's treatment of you. You need to know now, how you would handle it. If you don't know, then I suggest you seriously do some deep thinking about it. When it comes to those who are closest to you, there still must be personal boundaries or you will be treated badly.
If you do not know now, what the consequences would be if your boundary was dishonored.......why do you believe, by waiting, you would know how to handle it at the time? And what does setting a boundary have to do with her bringing up the R?
Perhaps you need to read Cadet's link on boundaries.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!