He thinks he did nothing wrong what do you think?

Never told me he was emailing his female lab partner when class ended.He figured I would never find out .and if i did he was doing nothing wrong.It was all about his nursing clininclas .It was her idea to stay in touch.He figured I would ovreact so why tell me ?I said it was disrespectful.What do you think?She is married we own our home been together 7 years at the time this took place.He was 38 she was 40 I was 42.