My penis don't gets straight and big when i see girl. so in future how can i sex with my wife because my penis normally don't get straight and big when i see girl. so is there any medication to get interest in sexing with my future wife?

Your penis is not supposed to get erect & hard when you see a girl. How inconvenient that would be and embarrassing. When and if you are in the process of partner sex, the complex interaction between attraction, your brain & your body should bring this function into action. If it does not, see your doc.

Seeing a girl and getting an erection is not the only method to get one. There are things like smell, kiss, touch, manipulation etc before you think of taking a medicine. You ar young and should have no problem. But if you are still worried go to a Urologist and get checked. Good luck