He cheated. They're married so even if he doesn't like his situation or he is confused about this new feelings he has for the boy, he should have respected that vow. But he went behind the brother's of the person he says he likes (the sister) back and slept with other women.
He has no honour as a man but, like other comments says, author says otherwise.
At the end, it depends on your views and morals.

I hope that it's just a misunderstanding because if he actually cheated am so out of this shit