Yeah, the parents are an interesting element.
Talking to my own mum, who was a teen at that time, I really think a lot of people were caught between two clashing worldviews. On the one hand, it wasn't uncommon for teens to be in many ways more like adults. People left school at 15 or 16 and went to work, might marry at 17 or 18. Younger teens were seen as relativly closer to adulthood.
In the older paradigm, the younger teens would have largely been protected, and even when they started being adult at, say, 16, would have lived at home, and their parents would have a lot of influence on who they saw. Sex was dangerous because you could get pregnant or an incurable STI. So the whole thing was serious.
Then all of a sudden there is this idea that sex is safe and recreational, and protecting teens who want sex is oppressive. And we can see now, there are really parents who just believe whatever the dominant progressive culture says, and of course some of the girls themselves were quite keen to be involved.
Even now plenty of parents will let their teens have all the sex they want so long as it's with boys close to their own age. Who aren't necessarily good people just because they are younger.