I'd disagree on one point - I think it is rude to publicly engage in infidelity, knowing that your partner will be hurt and humiliated when (and it's when, not if, in such cases) they find out.
I agree that the magazine was rude, in the terms of the regular world, for reporting on a private situation. However, this type of publication is part of the machinery that drives the entertainment industry these days, and generates those multi-million dollar profits. Gossip fills the coffers.
Perhaps it would have been best for all of us, back in the early days of the film industry, if they'd followed the tenets of the early producers who refused to name their stars - Mary Pickford was "The Biograph Girl", to her early fans, not a "name". But actors (and directors) have benefited financially from becoming celebrities. I'm not sure Stewart would give up her considerable earnings to be just an ordinary 22 year old girl in a love triangle. It's a case of live by the (fame) sword, die by the sword.