I went through the EXACT same thing that you're going through right now when my girlfriend and I first started dating. And what I did was, I told her that every guy is a lying cunt and just wants to try pick her up. Surprisingly we didn't argue about it and she did end up believing me after having a long think about it.
If you want to tell her like I did, just tell her the truth, tell her she is a beautiful girl and the only reason guys want to talk to good looking girls is because they wanna see if they can get in.