The phrase once a cheater always a cheater isn't always correct, the whole thing is about breaking a psychological barrier, meaning that cheating once moves you beyond an imaginary ethical barrier that you have placed for yourself, pushes you into a completely new comfort zone where cheating is acceptable,

take for example a college kid using a bong for the first time, how hard is it for him/her to make the decision of smoking it, now look at the kid a few weeks later, you will find him/her using it casually, its all about breaking the barrier for the first time, defining what's acceptable or not in your mind set,

of course there can be no generalization and people can re- establish this barrier for one reason or the other, if you caught your significant other cheating and stayed with him/her you need to make a huge deal out of it but in the same time not hold it over his/her head for the rest of your lives.