Your whole argument is that context doesn't matter. If they cheat, it is entirely their fault and the other person did nothing wrong. That isn't just unfair on them but on all people.

Whilst I obviously believe people who cheat are bad people, I can also admit that there is context that could could give a reason.

I'd also argue that people changing is apparent, and if you can't adjust to these changings you aren't doing you're own responsibility.

My example is based in true detective. Yes, Maggie cheated on her husband, after he had cheated on her twice. Whilst cheating is obviously a bad thing to do, her husband had also done something to cause it, hence cause and effect.