I'd like to further Katsuni's original point of there being three types of infidelity, and maybe find out more about why people, on average, feel threatened by these different aspects.
Well, I think this one's pretty obvious. All of the reasons that we've mentioned already in this thread, plus the evolutionary drive to protect our DNA line, make for a pretty offending thing.
Love being the only reason for a relationship is a relatively new concept to human society. People used to get together for a variety of reasons, survival included. A family unit was a much more efficient way of getting by (you know, teamwork and all that). Today though, we are fortunate in that many places in the world don't have to worry as much about these other reasons, so love/affection takes precedence in many cultures now. Given it's importance to a modern relationship, if there is emotional infidelity, it does pose the question of why the relationship even exists to begin with. One side is not giving the emotional support, the other is getting it elsewhere, so it's obvious that the bond here would be in dire straights.
This is the one I'm not so sure about in the context of a relationship. In a corporate/national environment, it would certainly be to the detriment of your company/nation if your intellect or ideas were being given to an outside entity/competitor. But in the context of a relationship, I'm not sure how that would translate. I'm guessing maybe a breach of confidentiality? I suppose this could be the most damaging overall if you set to ruin your partner, but I just don't see how or why that would occur if you had the physical and emotional aspects of the relationship in good standing.
Perhaps intellectual infidelity is a threat when coupled with infidelity of one or both of the other forms.
Just some food for thought.
Idle hands my dear, idle hands.when happy with who I'm with, I'm a model lover... I would never dream of cheating! I've cheated only once while in a relationship myself, but have been the other woman in a relationship before while single... in both cases I was bored