Marty should have parked his mom

Its the 25th anniversary of Back to the Future today, and I'm going to celebrate by watching it. But before I do, I want to clear something up; if Marty had sex with his mom, it would have made a divergent timeline and in that one she wouldn't have ever been his mom (I assume once she had Marty dick she's never bother with George again - once you go son, nothings ever as fun). So the original timeline may be a bit crazy, but technically this is just some random girl and Marty was dumb to pass up her banging 50's body.