Nelson is correct. Even married couples have problems with that. It is very sad.
Personally, I would not say it is discipline or morals, but respect. Love is just not having a deep affection for another person, it is having respect for that person. Some say, "Love is never having to say 'I am sorry'". That does not mean that you shouldn't say sorry, but not doing something to the other person that would hurt them is love.
Couples do not respect each other like they should. Out of respect for the other person, you would not cheat on him/her. If the other person in the relationship cheats on you, that person does not respect you. Without respect, there is no love. If they really loved you, they would not have done that in the first place.
Some might dis-agree with that, but that was how I was raised.
Tom