When it comes to Ennis and Jack I have no problem to call it cheating. They are both cheating on their wives, there's no way around it. And I think it's an important part of the story not to sugarcoat it. Both Annie, and perhaps Ang in particular, wanted to show us what happens when society forces people into playing roles, and to show how that impacts not only on Ennis and Jack, but also their families.
That´s why I think it´s important to recognise it for what it is and that it indeed is damaging to everyone involved.