Upstairs those don't indiscriminately give people conclusions, if he is underage, there are such ideas and impulses should belong to the normal, just lack of knowledge of sexuality, understanding of the conductive less, on the opposite sex have a sense of God's secret is very normal! Read more books on this subject, right? More with the opposite sex hooked up, do not panic, this is not your fault! It's a normal reaction of teenagers!
Of course, if you're an adult and have a girlfriend, and you're having these thoughts again, I'd suggest you go see a doctor, there's no shame in going to the doctor, what's shameful is that the mistake is going on.
No matter what? I hope that you will soon be able to go to release such a mentality!