Going to bed with someone else when you are supposed to be in and exclusive relationship is deemed to be reason for splitting up that nobody would dispute. This is taken for granted to such an extent that the promise of never being unfaithful seems to license to do pretty much any other kind of damage whilst still winning moral points. Unless the other person is screwing around you have to stay and work at it. All other means of destroying a relationship are to be endured.
This, of course, is ridiculous. The odd infidelity isn't the only way of creating the diminishing scars it does. That can be done by simply removing yourself from your relationship, however you do it. For example, sharing you inner most thoughts and feelings with someone else and not your partner can be just as damaging as screwing around.
We are all going to be attracted to another person at some time, but to prevent us acting on that attraction we have to feel that our relationship is worth preserving, that it would be insane to risk it all. That is why we feel so shattered when someone else is unfaithful. The message is clear: 'You weren't worth holding back for.'
But the pain of understanding that you are second to friends, work or another girl is just as bad, in some ways worse. The essence of betrayal isn't sex; the betrayal is turning their back, running away. At least sex with someone else is clear - it’s a misdemeanour that can be discussed. Yes, it hurts like hell, yes it’s still betrayal but it’s physical and not emotional.
Mutual fidelity is something most couples insist on, but why when we clearly find it so challenging? It is because sexual fidelity is synonymous with commitment. Commitment is intimate involvement at every level - sex is only one of them. It might be possible for infidelity not to mean betrayal. It is equally possible that removing yourself emotionally, that does not involve sex, to mean complete betrayal.
Having someone stay out all night, repeatedly sharing feelings with somebody else and absenteeism in a relationship still leaves that feeling of abandonment remains; the feeling that comes with betrayal.
The painful thing about someone turning away from you, whatever they may turn to instead, is that they are trying to escape life with you. The causing of emotional pain is the issue; sex is only symbolic of that.