at what point???

At what point are you cheating?? I have been e-mailing with this guy. He's cool and makes my day go by pretty quickley we have been chatting for about 2 weeks and get along very well via e-mail. That being said he is engaged, we discussed this in the beginning because we were on a message board and found eachother just for e-mailing becuase we were bored. But we have been emailing everyday and actually exchanged pictures. Which he says after exchanging them my fiancee would kill me if she knew. My question is this considered cheating I mean it's not going any where like we are gonna meet and it's not going to. But is it mentally cheating? I am trying to think if it would bother me if my significant other was e-mailing someone else....