If you look at the definition of "obsessed" it's something along the lines of excessive preoccupation in your mind.
Anime and fandom is a hobby. This means that you should only be preoccupied with it during your hobby-time.
If you're in school and if you can't concentrate on what the teacher's saying because you're thinking about your Yu Gi Oh card collection, then that's "too" obsessed
If you have a date with someone you're interested in and you cancel it because you'd rather watch a new DVD you just got in the mail, that's "too" obsessed
If you forget to eat meals, forget to exercise, or forget to groom yourself because you're too busy in theO chat talking to n00blets, then that's "too" obsessed
HOWEVER...
If your work/homework/social obligations/etc. are all finished and you find your mind preoccupied with watching an anime, then that's a healthy and happy obsession.
You should always try to balance:
Hobbies/Play/Relaxation
Exercise
Social
Work
Intellectual
There are ways to do many of these at once. For example, if you're still in school, work and intellectual are both about the same thing. Also if you play sports with friends, then you're doing exercise and social simultaneously.
Just remember that even if you're a super-otaku, that doesn't mean you should neglect the other parts of your life.
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.)