It was written much later (early 70's for Lucas, then 80's for movie), and I assume to make the protagonist a rogue type hero to other men. It wasn't a documentary.
It's not that it wasn't a possible (but rare) occurrence in that time period but that it was included in the story to make the hero more swashbuckling manly. I can't think of another reason to insist (Lucas) on including this in the story. It doesn't add anything except to character development. Her age wasn't even mentioned in the movie because, well, cringe. They left it out. Why? Probably because it was a family movie.
Most men weren't into dating children. Elvis kept his relationship with 14 year old Priscilla (and supposedly didn't sleep with her till they were married) because it would have seemed weird. People of the time thought Jerry Lee Lewis was gross to want a 12(?) year old. To be fair, she was also his cousin, but what I'm saying is that even if the laws (written and voted on by guess who) were late to change, it was still frowned upon. Men who slept with 11-12 year olds weren't considered heroic or manly except by others with similar tastes. It wasn't something you boasted about.
My grand-parents had to ask permission to date, then marry (here you had to be 21 to marry without parental permission. Adult men didn't have sex with 15, never mind 11-12 year olds AND unmarried, without it being frowned upon.
Anyhow, to me, the insistence of Lucas is what I find gross. He wasn't writing this in the 1920s. It was added as a slap on the back "You rogue, you! Getting away with something you're not suppose to do!"
I find we excuse a lot in general because it was "the time". Often, the difference between now and the past is that we can speak up now, instead of just "frowning". We seem to remember only that some things were done, or that some men got away with it, and forget the many who thought it wrong.
I may be reading much too much into this and completely off because I'm not from the US south. Maybe this was acceptable (encouraged) over there?