I liked this a lot. Especially when the main character realises what the actual circumstances are, resolves never to see the other girl, then admits to herself she would probably end up in the same predicament if she were indeed to meet her again. And then does not manage to make two full steps before stopping. Reason I like this is because my experience in life so far has led me to believe people rarely change, and even when they do, it is a slow, gradual process that has little to do with conscious choices. People often have 'epiphanies', and make 'resolute' decisions (much like the main character here), but their resolve often crumbles rather quickly, especially when the driving forces behind their actions are emotions.
The other girl, on the other hand, seems fully aware of the situation from the very beginning, and is quite unambiguously in it for the cheating aspect. I like that too, I like when a character makes no effort to repress a part of their nature, even if said part is awful in some way.
Interesting how a different presentation can make that much of a difference, the last story with cheating I read, I liked the story, but disliked the characters. One was an emotionally idiotic moron who did not seem to comprehend the situation fully, the other was a manipulative bastard (at least it seems so) whose motivations are not explored by the author.
Here, though, I like both the story, and the characters. Cheating done right, lol.