I posted my previous response before reading yours, but GMTA! You just said it better. :)
It's both, of course. As a woman I hate to say this, but Mrs. Edwards is overweight and ravaged by cancer. There have been numerous treatments and surgeries that had to have made her less attractive. I'm sure she has felt terrible for long periods of time as well, and John probably didn't get much sex at home. So because he is a person without integrity, a guy with an ENORMOUS ego, he thought he was entitled to screw around and get relief. Narcissists always excuse their sins because they're special.
Contrast this disgusting behavior with that of my brother. My sister-in-law has spent a lot of time being sick and had several scarring surgeries. My brother has stood by her unfailingly and continues to think she is the most beautiful and desirable woman in the world. Nothing and no one could tempt him away from her.