I think that we have to really bend the whole Butterfly Effect rule, where BTTF is concerned. After all, given how profoundly Marty's actions changed the trajectory of George McFly's life, what are the odds that his romance with Lorraine took the exact same course, and they had relations on the exact same night on three occasions, over five years? The Marty we know "should have" ceased to exist, resulting in a major grandfather paradox.
Basucslly, BTTF, like Star Trek, follows a "durable timeline". It's kind of like in "First. Contact", when all of Cochrane's support crew are killed by the Borg... but somehow the future is not affected.
Probably like the Farscape idea, where time is "elastic" and "can be stretched" to an extent without snapping as long as you try to get things as close to the original circumstances as you can.