Yes, I think a boundary would be 'I'm not willing to invest in saving our M if you plan to see other people.'

The message is - you're free to do and see whomever you like, but if you do X, I will/won't do Y. It's all about exercising your own free will in whatever situation.

T 13 M 7
Me 48 H 46
SS 15
BD 7.14 PA
D final 5.16 (H filed)

We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus