Plothole in back to the future?

Is this ever explained?
In the movie Back To The Future Marty goes back in time, messes things up and then needs to make sure his mom and dad still fall in love so that he can get born in the future and doesnt stop existing.
Ive been wondering how his parents or anyone else for that matter didnt recognize him in the future?!?!? This random guy Marty just pops up out of the blue and then disappears just as suddenly, they never see him again only for their son to one day turn out looking exactly like him. That doesnt make any sense what so ever.
I still love those movies though.