[Bells]

It's just the thrill of the challenge to be honest. Also the fact that you're with another girl might make them think they're missing out, so again that adds to the challenge.

But if you did cheat and go to this other girl, chances are she won't be the same, because the thrill of the challenge has gone. Not to mention she probably won't have as much respect for you because she'll probably expect that you'll cheat on her with another girl and so on (once a cheater, always a cheater and all that). Then you'll be left with neither of the two girls. So it is good that you're faithful.