Here's some empirical observations on faithfulness in human beings. By faithfulness I mean monogamy-you not cheating on your partner. I believe it is genetic.
This is valid only with humans who are free to choose their sexual/love partners-which means primarily in the Americas and Europe. Asian countries which support institutions like arranged marriages are not a part of this group.
I have known a lot of human beings in the Americas (North and South) and many people form Europe as well. I have tried to look at human beings just I would look at any other animal; and even though it is hard sometimes, I am getting better at it.
I believe that there is a percentage of population, somewhere between 5-20%, which is GENETICALLY faithful. These men and women do not cheat on their partners. They may practice serial monogamy; they may have lots of amorous relationships sequentially, and even have orgies when they are not attached to anyone; but when they are in love, and especially when someone else loves them, even if they are not in love, they will not cheat on their partner. They cannot cheat on their partner, even if consciously they want more sex, more variety, etc. I believe this is genetic; there is a gene or a set of genes in these people which makes them behave this way.
This faithfulness behavior is similar to homosexuality or left handedness-two other characteristics which are also genetic in my opinion.
There are some Swedish researchers who have found evidence in this in some animals. However, my observations are empirical and are limited to this variety of human beings living in the Americas and Europe. The rest of the population in this group, 80-95%, is not faithful; they are sleeping around even when they are married or have a boyfriend/girlfriend who is ostensibly their formal partner.
I can give some examples of people I know quite well who I know are genetically faithful; maybe some biology researcher has an interest in finding out what's common in the genome of these human beings.
See also: Genetic affinity in Friendships