It's natural to be attracted to people who are sexually mature (I mean able to have sex/displaying physical sexual characteristics. I'm not sure if that's the best way of phrasing that, but you know what I mean), and a lot of teenagers are. We are designed to be having sex and having babies after we start puberty. Whether teenagers are emotionally mature enough for a sexual relationship is another matter, which I suppose is where the moral/legal side of it comes in.