The Oedipal plot also feels harmless because we only get half of the Oedipus story—just the mother-son stuff, not the son-murdering-the-father part. Oedipus kills his father, Laius, at a crossroads. Marty saves George on a suburban street—a reversal, whether explicit or not, of that moment. Furthermore, Marty doesn't want to replace his father; on the contrary, he spends much of the movie trying to unite his parents.
Which brings us to the other weird thing about Back to the Future. Most people avoid thinking about their parents' sexuality. We accept that our parents have copulated at least once (or twice, or however many siblings we have) in the distant past and then drop all further inquiries. Marty can't drop it. He must involve himself directly with his parents' romantic lives in order to ensure his own existence.
Nowhere is this involvement, and its attendant strangeness, more blatant than at the Enchantment Under the Sea school dance, where George and Lorraine are supposed to share their first kiss. Up on stage as a replacement guitarist, strumming away to "Earth Angel" with Marvin Berry and the Starlighters, Marty watches his parents dance, and hopes that the kiss will actually take place—a guarantee that history is back on track. Stuck in the guitar strings there's a photograph that once depicted Marty and his elder siblings and that now depicts only Marty. The conceit is that if George doesn't kiss Lorraine, Marty's image will disappear, too, signifying that he'll never be born. If he does, Marty's image will remain intact, and his siblings will reappear. It's a clever visualization of an abstract drama, which also reinforces that Marty isn't merely waiting for his parents to lock lips but waiting for them to have sex and conceive his older brother, his older sister, and himself. He's eagerly anticipating a primal scene.
Watching your parents have sex (symbolically, at least) not once but thrice should probably result in severe trauma. Sigmund Freud thought that one of his more famous patients, Sergei "Wolf Man" Pankejeff, had witnessed such an episode (a tergo, no less), and blamed the event for his future neuroses. Of course psychic distress isn't especially appropriate for young moviegoers, so Marty feels none. Nope—after George kisses Lorraine, Marty's so ecstatic that he leads Marvin Berry et al. in a rousing rendition of Johnny B. Goode, which culminates in a rather bawdy, suggestive guitar solo. His parents' (symbolic) orgasm leads to his own. Otherwise put: Marty (symbolically) masturbates to his parents (symbolically) having sex.
Back to the Future was hardly the first science-fiction narrative to use time travel as a way to sidle up to incest. In Robert Heinlein's Time Enough for Love (1973), the protagonist "jumps" back to 1916 and falls for his mother. Unlike Marty, he actually consummates the relationship. What's unusual about Back to the Future is that it takes the stuff of Greek tragedy, and psychoanalytic case studies, and crams it into a PG format—by telling us there's really nothing to get upset about. Parent-sex isn't disgusting; it's joyous. Without it, we wouldn't be here.