It's not like the MC was cheating on his wife.
In the other hand, her daughter wondering if she should forgive her father, while she's fucking around with a boy, who her father barely knows.
Think this way, what if the MC fell on despair and commit suicide, then his life, his daughter, and Koto's life practically screwed over.
Indeed Koto has an issues with her mother, abusive mother that is.
And what could be happened, if the MC didn't save Koto from rape incident hmm?