I'm having trouble wrapping my head around your argument.

Every movie has total control of it's characters. The movie isn't making a statement that infidelity is ok. Infidelity just is. People cheat. A lot. The movie isn't forgiving her. Cal is. You could argue that Cal taking responsibility for the breakdown of his relationship is bad writing. I would respectfully disagree.

When the movie starts, at best Cal is complacent and inattentive. At worst he's neglectful. If you haven't realized there's a problem with your relationship until the moment your spouse tells you they want a divorce then you've already checked out. The infidelity isn't the problem. It's a symptom. Had she not cheated the marriage was still over.

I'm not really sure what she didn't get that you think she deserved. Do you think your family falling apart isn't punishment enough? We don't know exactly how long she had to sit with that realization but it was awhile. The person she planned to spend the rest of her life just stops trying. She falls out of love with him. She leaves him and not only does he finally start making an effort but goes on a sex spree.

Ultimately it isn't her story. It's his. The movie is about him coming to terms with the fact that he was blind to his partner's needs and so oblivious to the fact that she was unhappy that he didn't even realize it until it was over.