Infidelity!

It has been a while I am thinking we are all following the norms that have been set up by our ancestors long time ago when there was no trace of us! We have not consented to the validity and credibility of so many of the social norms that we have to obey. Based on the knowledge and context of hundreds of years ago, the current norms have been shaped and we all acknowledge to the fact that the knowledge of those days in amount and depth was in no way comparable to that of today, thanks to all the scientific enquieries.
One of the most family splitting issues in the world has been Infidelity! As far as I know there has been very little attempt to precisely define and delineate the boundaries of this sinister act! What is infedility? What could and what should not be considered infidelity? What if someone can really share his/her love with more than one person at the same time equally? What if someone can share the happiness with more people?
This question and any similar questions have been so brutally treated that we never dare to even think about them.
Another piece of argument which at first may not seem relevant is: why is infidelity the main family splitter when there are many other instances of behavior which are far more flagrant? For example: aggression, discrimination, humiliation, tyranny, physical and mental abuse, dictatorship, dogmatism, selfcenteredness, and so on. You acn easily and sometimes even proudly openly confess to having some of those behaviors with no shame, however just the talk of showing affection of any degree to a third person is so destructive. Who decided for that? Why is there no or little tolerance for spreading love while anger, wrath, gealousy and animosity are so easily spreadable at any degree with a word of simple apology if we need to gain even more proud! Should we rethink our social practices?