I think with emotional affair, it's beyond a simple platonic friendship. It's when the emotional needs of the couple become neglected or I even consider going to advice or to vent(not basic advice but to rattle on as though not behaving solidified enough to confront and communicate with one's own partner) about one's partner with the opposite gender to be teetering on a slippery slope. Because then the energy and commitment needed to maintain a healthy relationship is lost.

Could I, would I forgive cheating? Eventually forgiveness is about relieving oneself of bitterness from betrayal. So in that sense, of course. I'm too weary to carry such weights on my shoulders.

As far as staying or leaving? I'd give committed therapy a chance provided it was both being truly committed to that. A boundary of trust would be breached and it's either permanently destroyed or would take a specialist to get to the root of the matter. Things would never be the same, no doubt.