The above is a common act in this day and age, so much so it isn't even classed as a negative act within many circles. In fact in many ways infidelity has become an unofficial sport.

People nowadays cheat on their partners for all sorts of reasons like boredom, negligence, abuse, fear, insecurity, dissatisfaction, and so on. 

Infidelity is a classic example of a man/woman trying it eat his/her cake and have it. This is being said simply because if an individual is unhappy within a relationship then the logical thing to do is to leave the relationship before moving on. But people refuse to do this because they want their satisfaction be tendered to and at the same time embrace the security of an official relationship.

Infidelity of any kind is an unfair act, because it is a sign of selfishness that is extremely destructive within a relationship. It is a sign that the infidel in question is incapable of loving his/her partner, because he/she hasn't even begun to learn how to love him/herself, which is why he/she was able to get his/herself drawn into the realm of infidelity for whatever reason.

Infidelity within a relationship should not be accepted or tolerated, because a man/woman that can cheat on you has the potential to willingly destroy you. 

This is free advice, do what you want with it.