Yes it is a breach between man and wife. Man and Allah as well. But not all cheating is done for sex. I am not defending betraying the trust but sometimes there are very valid reasons for the breach of trust. In the case of someone I knew from college. He got married to a girl he dated in college. I am not sure what the exact situation lead to the marriage, but within the first three years she cheated on. That lead to a seperation but they got back together again. Then something else happened. His mother was sick and his wife blew all of them off for vacation.
Eventually he started to confide in a coworker. It developed. Cheating occurred and he divorced his first wife and now is very very happy with the second one. He is a happier man for it. In that regard, is it truly a bad thing that he found happiness from a horrible marriage?
Now before you women go crazy. Take my story and change the gender to a woman in a bad marriage. Would you still be considering it wrong?