People cheat for many reasons, some of them not as vile and scummy as so many of you so readily brand them.
Some actually 'cheat' consentually, as a sort of roleplay. The 'thrill of being caught misbehaving', despite the partner already agreeing to it taking place ahead of time. It's basically an open relationship that both sides pretend is closed, just for the fantasy... or something like that. Some develop like this when it becomes apparent that one partner cannot fulfil the other in some capacity.
Paralleling that, many people actually know their partners are cheating, yet accept it because at the end of the day that partner still comes home to them. Whether they go looking for sex, or whatever, it's the being together that keeps them together.
Then you have the abusive relationships where, for one reason or another (or perhaps many reasons) the 'cheating' partner is unable to outright end the relationship and so sneaks off to seek some sense of happiness elsewhere. In some cases, this later gives them the courage and security to finally end the nasty relationship and escape to the better one - Sounds stupid, but I've known a fair few that went this way and if the 'cheat' had instead just faithfully remained in the toxic relationship you'd all be branding them cowardly idiots, I expect...
Having never cheated on someone myself, as well as having been cheated on, I am instinctively on the same high horse as many people here. But having also learned some of the many reasons people do this, I'm no longer so quick to judge them all based on just my own very simplistic, monotone morality.