The "ZIP code rule" establishes the scoundrel's primary philosophical question when it comes to monogamy: Is it cheating if it happens in a different ZIP code from the one occupied by your S.O? And all too often, the conclusion is: "What they don't know won't hurt them." As a bonus, if the S.O. does find out, he or she will likely be too far away to key a car or smash some plates. A cad will behave like a cad no matter what, but the chances are perhaps greater when his or her S.O. lives far, far away.
Monogamy can be a challenge over time even under direct supervision. (Of course, ideally, direct supervision isn't a requirement for monogamy.) Most people, though, are either the cheating sort or decidedly not. But some people who would normally stay true to their S.O. don't respond well to long-term physical separation. Loneliness creeps in, new and interesting people appear when you least expect it, and then there's alcohol. More than a few bad romantic decisions have been made under less-than-clear-headed circumstances. (It also presents a twist on the ZIP code question for deceitful boyfriends and girlfriends everywhere: "Does it count if I barely remember it?")
Distance, loneliness and alcohol often grease the tracks for the derailment of a long-distance relationship.
This leads to our next reason long-distance relationships just don't work: a lack of trust.