In Back to the Future 2, Marty and Doc travel into the future and see him older and with children. This could not have happened. When Marty left his house with Doc at the end of 1 / begining of 2, he left. He was gone. No more Mary McFly until he showed up again in the future. In the first movie, Doc sent Einstein one minute into the future. WHen he did, the dog was GONE for a minute. So Mary would have been GONE for years.