Haha, I used to be just like you when I was at school. 17 and 20 isn't such a big difference, women mature at an earlier age than guys and this is just natural. There was a few years age gap between my parents and they were married at 18 and 20, Mother, Father respectively.

Now I think it'd only be a problem if the girl was under 16, that's it. Otherwise, she's legal and it's her choice.