In my mind, cheating is considered to be a situation when a partner is close to some extent not only to one person. Keeping that in mind, I am thinking - how is that supposed to be a betrayal, how is it supposed to affect the relationship negatively? There is the answer - it is often impossible for a person to give the same amount of attention to the partner if he or she became close with another one. It also gives the person who is cheated on hope and great time, but by cheating this hope slowly dies and there is less great time between the two and less passion in sexual life. Thus, this hurts the person who is cheated on. However, it mustnt be always true that there is less attention and passion given. This way usual break-ups must be considered as cheating as it hurts the person who is dumped much much worse.
What is the real reason? I came to a conclusion that it is unconsciously considering your partner a property of yours, childish behavior "I want everything from you, you are mine and no one else's, YOU ARE MY TOY". If your partner cheats on you, it is an insult for you, but it does not hurt you in the first place, it hurts mostly your ego. You are being complimented by attention, you feel significant, it is a super ego boost. If you never find out about cheating and there is no effect on your relationship, everything will be fine as the ego is satisfied. BUT IF you get to know about this, cheating is bloody betrayal and "I am breaking up with you" and all that shit. However, the relationship should have stayed healthy until the break-up.
That's why close friends and lovers become distant. Because of the freaking ego. People who love each other and have one of the best relationships in their life become rival to each other because one of them thought he/she was his/her property and only he/she could own the partner, otherwise they consider it as a betrayal. Pride, self-esteem and all that shit prevent people from passion slowly dying away with mutual respect and friendship if one of them finds better love or accepting temporary interest in someone. Instead, there are lots of single poor girls and guys with broken hearts. And there are even more couples controlling each other not to have any freaking contact with the opposite sex which is a shitty relationship, to say the least.
This convinces me even more that most of us are just grown up kids. Childish behavior is never accepted in healthy society as this affects other people negatively, we learn to limit our ego all life not to hurt other people. It's time this approach to be applied in private life.
I am looking forward to someone who will reassure me that cheating is a complete horror.