Welcome to Healthcare Magic This is normal occurrence. Not to worry. It is not harmful and will not cause impotence. It happens especially in adolescent age group. Unless it is causing problem to you, nothing to worry. You need to avoid thinking of such things by displacing that thought by other thoughts by something you dud at study or work, something funny that happened recently or something that needs your undivided attention. This way you can give your time to more important things and avoid this discomfort. It is best to avoid doing any thinking at night and try to get refreshing sleep. Spend more time doing productive work like in your studies or work so that this opportunity does not arise. When ready you can have relations with a boy when time comes. Till then focus on your work and time will come soon. If these thoughts are not controllable you may need to consult Psychiatrist to help you prevent this from happening and get good tips and therapy to do so. Eat healthy. Exercise daily. Avoid stress. Do Yoga, Pranayam to relieve from stress.