Therefore, my question remains, "Why does sexual unfaithfulness break people's heart to that point when there are no feelings involved?"
I think...fear & jealousy...i.e. a person's marriage 'security' or that of the children's is at the center of it. They fear, possible loss of everything they have invested.
But, I think, too, besides what has already been said about 'religion, society, culture, sometimes even the law', affecting how we are 'supposed to feel', something may/may not be lacking in that relationship. And, being human, doesn't that make ALL of us lacking something? Whether it is just a good conversation, intellectual stimulation, physical attractiveness, physical stimulation or sex, etc. Can ALL of us possibly be ALL things -- even to our mates?
I don't think so.
To me - the BIG thing would be -- deception. Deception is what will loosen the relationship glue.
With me, now (post JW) I can be honest with my super non-jealous 'worldly' wife, not at all like the JW xwife. Someone mentioned how jealous they get if their man even looks at another woman, why? Why are people so scared of their man or woman appreciating a passing beauty? How many of us belong to the abnormally 'Beautiful People Club'? Not me. The abnormally beautiful are doing movies and TV, selling or modeling underwear, etc.
When we walk by or see nice bodies... just in the unlikely case I missed it, she points out to me "...did you see the the red-head?" And, I know how much she loves to see a nice shaped guy's butt, if I see one, I'll tell her where to look. That...is called confidence and trust.
Lastly, we always say, "my husband, my wife...", right? It almost sounds like we -- OWN -- the other one. We don't.
I think it should be up to each couple to be totally open-minded to one another. They should decide what is possible and acceptable in their own relationship. Deception will destroy that...