I had thought about this topic over the winter when my then fiance and I had been about 15 months apart and still had 8 months before our wedding, and it has just recently come to my mind again.
Now, there is never a good enough reason to cheat, ever, for cheating is taking that which is properly due to one and giving it to another to whom it is not, and such a thing is an offense against justice. That being said, I have come to an understanding of how some may come to find themselves accidentally in a cheating situation, especially if they are in long-distance relationships. There are different kinds of cheating, broken down most basically into emotional and physical, with physical being the one which most identify as cheating. (Again, a little disclaimer, sexual infidelity can not be accidental. Sex requires an act of the will, an act of consent for one's body to be acted upon in such a way, which is manifestly different than one realizing that he has had one too many without intending to.) Most cheating situations, at least for women, I think, begin with the emotional and culminate with the physical, and are a result of loneliness which they may not even know existed at first.
To begin: the accidental cheater does not realize what they are doing at the start. It most likely starts with a friendship, probably a new one. This friendship is new and exciting, intriguing and interesting, as most unknown things which peak our curiosity are. Being friends, they spend time together which is not a bad thing, and it is good to stay social even with one's loved one so far away. But the absence of the loved one, the unavailability leaves the cheater with a hole, an emptiness in themselves which it is hard not to fill. This new friendship does something to ease the burden of loneliness and is innocent enough.
Slowly, the two new friends find themselves more drawn to each other, for they get along easily and understand each other well. Oh, there is no danger of anything happening between them, for it is all very well understood that the one is taken. The cheater still talks to the beloved every night, though with slightly less enthusiasm than before, and begins to more eagerly looks for the next time they will interact with the new friend. The beloved-shaped hole in them has been filled by this new friend. No, it is not an exact fit, nor does it seem to fully satisfy, but soon enough they are talking of matters, emotions, thoughts which are proper to the intimacy of lover and beloved, which they are not. Such mental and emotional intimacy, if left unchecked, will eventually lay claim to physical intimacy as well.
Thus has the cheater robbed their far-off beloved of that which was due to them as beloved: both mental and emotional intimacy, which by nature, further demand the completeness of physical intimacy. It is easy to end up in such a spot without realizing it, for what could be more good and desirable than to have close friends? Especially when one is lonely? How hard it is to keep that beloved-shaped hole unfilled! How difficult to say no to companionship when man is by nature a social being! Nevertheless, such situations must be guarded against, lest one end up as the cheating kind. But I do understand how one may be accidentally a cheater.