In all honesty a girl that tries to get a guy to cheat on a girl that he is actually committed to is nit going to be respected... at all. I'll think she is easy, shallow, and I'd never consider to be with her. I might have a one night stand with her when I'm single again, but as far as that, I'd be pretty much done with her.
However, you didn't ask my opinion and I have been the guy in the situation. If it's strictly sexual and not because you want a relationship(because it probably won't happen, it won't be a serious on, and he'll most likely drop you for some other girl faster than he did the other girl) I suggest by starting to point out his current girls flaws, not just physical. That will do nothing. If she nags him a lot tell him. Tell him he's whipped from time to time. Tell him you wouldn't have those flaws. Figure out ways to get him alone more. since it says under 18 for you age it's safe to assume you're in high school, get him out of class. If he's willing to spend alone time with you then he's willing to cheat.