Earlier quoted context omitted.
As though the thoughts and feelings of another person are as predictable and consequential as the laws of physics. I wish.
I agree that the thoughts and feelings of other people in general are difficult to predict. But a person you marry is often someone you spend a lot of time around and hence whose thoughts and feelings can be predicted to some extent - because you see them in different situations, then see their reactions and talk to them about their thoughts and feelings. In this specific case, the author denies that the consequences…
I did a bad thing - well I didn't intend to do it, so I'm still good/right.
Someone else did a bad thing - they are a bad person.
I should be measured by my intents, not my actions or outcomes.
Others should be measured by their outcomes, because thats obviously what they intended.