I agree that each relationship is individual-but there is overlap. How much overlap, depends upon the people involved.
It would be flat ludicrous to suggest that problems in one of my relationships won't impact the other-we all live together and we are raising children together.
THAT SAID-when Maca had a gf who was toxic for the rest of us-we simply declined to have anything to do with her. HIS CHOICE when/where/how he went to spend time with her. But-no bringing toxic people to our home.
As for NRE,
I quite sincerely have no patience for it and I don't abide by the "let your partner enjoy it" adage either.
I think any person who gets so googly eyed over ANYTHING that they neglect their already agreed to responsibilities is being irresponsible and deserves to suffer the consequences of that choice.
I don't allow myself to neglect my responsibilities and I expect my partners to manage their emotions so that they don't either. If they can't do that-they need not remain my partners-and that includes my husband. It is in fact the experience of watching him fling himself into NRE and make everyone miserable-only to repeatedly realize he had nothing in common with the various women he did this with-that led me to "FUCK OFF-you have responsibilities and you damn well better meet them before you go running off to fuck" attitude.
Which has stayed with me and probably always will.
"Love As Thou Wilt"