Depends. Some relationships are open, and it's not cheating. However, in many (Dare I say, most) there is a common agreement to stay loyal, which also has an extremely strong genetic component. Yes, monogamy is programmed into our genes in many ways. Of course, so is having lots of sex, so that doesn't mean that we can't overcome certain aspects of our genes in order to have what we consider a better life. Essentially, whether cheating is possible is a function of the individual couples choice.
Beyond that, it is everyone's fault...to some extent. And it depends too much on individual factors. The cheated partner could have been an abusive, distant douchebag whose partner desperately needed some sort of love in their life after living so long in fear...or they could have been a sweet, kind innocent partner who for some reason didn't quite satisfy there partner completely without them even realizing it. The Cheater could have been the aforementioned desperate, abused victim...or they could be easily board, and wanting to have casual sex with anyone, no matter who they hurt in the process of their own appetites. The Other Partner could have not done enough research to realize that there new partner was already taken, or they could have been a close friend who needed to support someone they care about through a difficult time...or they could just want to get laid, and decide that the most convenient pool of potential sex friends would be the significant others of people they know already. Every situation is different, everyone shares the blame (Though sometimes, 1 or 2 partners share of the blame is incredibly minor) and often times, it's not as clear cut as we would want to believe.