I have to agree with anybody that says that this doesn't say anything about Lucas, but the kind of character that Lucas wanted Indy to be, and the kind of movie that he wanted Raiders to be. It would have turned "You broke my heart" into "You ruined my life" and would have severely altered the whole tone of the film. I can just imagine Marion being completely distrustful of Indy, not going anywhere with him, not wanting to be near him, but having to be because he's trying to save her father. And then there is Indy, feeling incredibly guilty for what he has done, feeling that now might be the time to make it right, if he can make it right at all. Granted, Raiders would not have been the sweeping adventure we all know and love: it would have been a dark and deep human drama, one that I don't think audiences were ready for then and would obviously cause no small amount of controversy even now, but a pretty deep movie none-the-less.
Maybe make Marion thirteen-fifteen, and make Indy in his early twenties (21 or 22, still young and dumb enough to do something like this) to narrow the gap and make it a little less creepy, but I could have seen this movie working. Though, I am glad they went the route they did and we have an awesome adventure movie instead of a disturbing examination of adolescent sexuality and mental trauma. I say this was actually a good idea of Lucas', just one I'm glad, in hindsight, he didn't pursue.