The OP has reduced this 4 decades old movie to a collection of diverse sexually charged situations that are treated as equal to each other. It’s an extreme oversimplification of a plot that owes much more to the absurd paradox of time travel than the ever present topic of sex.

Marty’s conceit to use assault of his mother as a way to correct the timeline and prevent being erased from the future isn’t an Oedipal trope for Marty, It’s a desperate but ill conceived idea that blows up in his face further complicating his situation instead of correcting it. At no time does he desire sex with his mother.

The subplot of Marty’s mom suffering from an acute case of teenage hormonally induced interest in Marty and his briefs, is introduced but then dismissed. At no time does the screenplay give any indication that she actually peeks at the goods. Despite that, the OP raises the issue to bolster the narrative then dismisses it as too complex to get into.

Biff’s character is the only one who appears to have physical sexual assault in mind. It’s an extension of his two dimensional character’s focus on getting what he wants whenever he wants it. This character has been around since before recorded history and lives with us today. It’s a reflection of the baser animal nature we all still walk around with. As human beings, we relate to the desire but most people don’t act on it because we’ve evolved to consider its destructive impact. Some haven’t. The moral of Biff’s story is on display when Marty returns to present day and he’s no longer the “man” he was.

The movie is a collection of stock characters placed in absurd circumstances resulting in a comedy of errors. A formula for entertainment since way before Shakespeare.

It is interesting that the OP didn’t reflect on the fact that the story doesn’t only live in the past. The musical adaptation of the movie first appeared in London in 2020 and has been running on Broadway for about a year. Its plot hasn’t changed much and continues to attract thousands of audience members each week. So the story lives on today.