Infidelity - often known as "cheating".  By definition - it's a

violation of the mutually agreed-upon rules or boundaries of an intimate relationship.  For some people, this means the end of a relationship, a "deal breaker" if you will.  However, for some couples it has served as opportunity for growth with the help from a licensed counselor such as myself.   Folks often find during and after the break up that there's more at stake than they bargained for. Especially with married couples and even more so if children are involved (time invested in relationship, finances, loneliness, depression, emotional ties, family, etc).  Married or not, if you're both willing to seek counseling it would definetly help to explore needs as well as possibilities for the relationship going forward.   Counselors like me can help, even if you'd like to seek counseling individually.