I think that everyone has kind of an "ideal" partner in their head, though most will end up with someone they absolutely love who doesn't fit that initial ideal image.

You probably lust over fictional men because the ideal partner you have in your head doesn't correspond to any man in your social circle (and no man in your social circle interests you romantically to the point of changing your ideal image to correspond to them). Characteristics of this ideal partner happen to match a fictional character and so you become obsessed with him.

Don't worry, once you meet the right boy, I think your lust will switch over to him, though you might always give the fictional character a special place in your heart. The important thing is to not let your fictional character lust interfere with your pursuit of Mr. Right. He might get turned off if one day he sees that your college dorm room is like an alter to an anime or video game character. (Though maybe, if he's an otaku, he could cosplay as him for you.)