IMHO, if someone is open and honest about their intentions and what they want there is no need for cheating. There are more than enough people out there that are perfectly happy in having open relationships and in fact prefer things that way. But, if you are in a committed serious relationship and it is understood that things are exclusive and you cheat, it's because you are spineless and lack character and virtue amongst other things.
I think that sometimes people get in to a position where they don't necessarily want to lose someone, and maybe they don't want to be alone, which isn't necessarily the best way to be, but, whenever an opportunity presents they cheat because they want to be with another person, or other people, but they don't want to be alone or cut lose from what has become familiar.
my .02 I didn't put much thought in to it, kind of incoherent thoughts and ramblings.