These are normal thoughts, especially at your age, and very hard to overcome.
Your problem is avoiding these thoughts in the first place, right? Try "replacement" thoughts for a while. It's very hard at first, but gets easier over time. As soon as you start having a thought you know is wrong, force yourself to replace it with a bible verse you have memorized, or a prayer, whichever one is easier for you. At first it might take enough concentration to re-focus that you have to stop walking, or shopping, or whatever it is you are doing, but do it anyhow. Every single time. And don't be suprised it it takes an hour to walk a few paces at first, lol. But the thing is, once you start getting used to it and replacing every bad thought with a biblical one, the bad ones start losing all of their attraction and eventually start becoming repulsive.
Also, remember that your future wife is out there, and you don't want other guys looking and thinking the same thoughts about her as you do their future wives!
Chrys