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See here, you need to understand that Erectile dysfunction
is failure of sufficient erection for SEXUAL INTERCOURSE. If you sit by yourself and expect to think up an erection it not necessarily has to be a full erection. Please get your idea right. Avoid spending so much time thinking these things at your age. You should concentrate on your studies, work and family. When you have sexual intercourse you will do just fine as you will receive the sufficient stimulation you need for a full erection and no need to imagine anything with effort to get it.