I had the same problem with my current and first real gf (5 months now) and I figured it would happen, so I was prepared for it. The first time it happened, I told her before she could realize on her own, and said that I can't help that I love her and she turns me on! Sometimes it happens still even now, but ususally when we're against a wall making out or something, and she generally likes to grind against my jeans when it happens. So eventually it'll become a good thing. Also, if it ever happens and you don't exactly need / want it to, pull your pants down a bit, loose on your hips, and hopefully you're long enough to tuck it into the waist of your pants. I'm 15 yrs old and 7 1/2'' long..so this works out pretty well, cuz it's hard to hide otherwise.