I mean.

The only thing worth mentioning about the first girl was that she was hot, apparently. OP literally made a point of pointing out that this other girl is not as hot as the first girl without a single other mention of issues with the first girl or their relationship, but their attractiveness is apparently the one thing worth noting about either of them in OP's eyes, so Idk what else to assume other than it being some kind of emotional attachment to the other girl that drove him to break up with the first. Like I said, emotional cheating may very well not be the exact term, but in the moment I couldn't think of something that conveyed the issue better.