I don't think cheating is the be-all, end-all horrible thing for relationships. I would be hurt if the boyfriend cheated - but because he didn't trust me enough to communicate what the actual issue is. It depends on how long the extra relationship went on, what he feels for the other person, and if they'd leave the other person no questions asked, but it's pretty unlikely I'd leave just because someone cheated. After we strengthed our communication again, I think it'd be time to figure out what the underlying issue was and see if we needed a new type of relationship though.
I'd be more concerned about the possible STDs than any sort of emotional thing.