TD, essentially, whatever relationship type a couple decide between themselves is entirely up to them, including one partners acceptance, or not, of the other partner reneging on agreements/promises/boundaries.
I know from personal experience that this form of deceit and any other would be deal breakers for me. It would be the height of selfishness and arrogance for a partner to have any expectation of a continuation of a relationship, if they chose to change the terms of it.
It isn't about moral high-ground, I think it's realistic to expect a partner to fulfil an agreement. If they can't, they are welcome to leave, after all!
I believe it's incredibly unhealthy not to mention unfair for children to become entwined in webs of lies and deceit and sometimes from relatively young ages, just so that their 'adults' can get their rocks off, as and how they choose.
SGB, maybe they just care? If you knew a friend was being duped in any scenario, would you just stand back and watch? If you thought a friend was being duped, would you tactfully attempt to ascertain whether this was the case or turn a blind eye?
I do think some people step in to stir things up but for the most part, others do so with genuine intentions. Besides, if the potentially unaware partner is in fact aware and accepting of their situation, what harm does the raising of the subject by an outsider cause?