Cuz if Marty got her pregnant, she never would have hooked up with his father and Marty would have never have ended up being born or hooking up with his mom. And if he doesn't hook up with his mom cuz he never was born/went back in time that leaves her free to hook up with his father meaning he would be born, go back in time and have an incest baby only to not be born and not go back in time, etc etc