Is it wrong while in a committed relationship?

I live with my boyfriend of many years own our home.He went back to school for nursing.He had a lab class to start before he even got into nursing.He had a female lab partner he never told me I guess I never asked.When they were in class on a saturday he went to study at the lab needed teh microscopes.She showed up was planning on spending the day there.He was already there for a hour .So about 30 min after she showed they took a break she offerd him a pb and j sandwhich and some chips.He took it ate half he doesnt usually eat others food he is funny about that.So iam surprised.She is married by the way.She goes on a crusie with her husband and inlaws and ask him to mail her some school papers she wouldnt be there.So he did I didnt know she did pay for postage .Classs ends she wants to stay in touch ask for phone number .He says girlfriend wont liek that.He is not a phone person to begin with.Ok week later she really wants to see if he gets in nursing program and see what clinincals are like.Can I have your email so he gives to her I didnt know.After a summer class she e mails ask if she can buy his book they meet in grocery store parking lot and sells book talks for a few min about the class as he leaves she hugs hims.They e mail for about two years every few months about his clinicals,she talks about her family he is all about school i never know.Emails every two to three months she offers her phone number if he wants to talks.He doesnt even write it down.See we work together him and I and he is not friendly or out going or social so iam surprised by this.He said he was just being nice.And with email you write when and if u feeel like it you can bullshit also like when she asked a detailed question twice and he didnt feel liek answering and said maybe they can grab abite to eat sometime and he can tell her all she wants to know .he said this since he didnt feel liek writting a book.He never did meet up with her again after the sale of the book.HE stopped writting her once she lost his email .HE didnt want to write any more.But then she sent email to the college and they forward to him.I found it .I went ballistic.Felt hurt like he was cheating he said i would of overreacted or told him no not to write.i feel hurt .What do you think how would you feel?I really need to talk to someone about this in more detail.