The movie's very much an artifact of its time, salvaged by unusually sensitive performances from de Wilde and Lynley. The trouble is they look so angelic and innocent, and from such solid middle-class backgrounds, it's hard to picture them "experimenting". But then, I guess that's the point for audiences of the day—yes, it can happen to even those who appear least likely, so be on guard. At the same time, the two look so much alike, cynics might suspect incest.
As a teen from the time, this youth movie strikes me as one of the few made more for teen girls than boys. No speeding cars, beer busts, or other staples of the drive-in crowd (note also how demurely Janet {Lynley} is dressed). Instead, the 90-minutes deals with a subject that can't even be mentioned on screen, viz. abortion. People can be slaughtered in movies, but screenwriters don't dare even mention abortion. Thus, the mores of the time are much in evidence and reinforced by Hollywood's boycott-fearing Production Code
For example, no mention is made of contraceptives, sex education in school, or safe legalized abortion as possible alternatives (note how the abortion escort is made to look like a witch), since one or all of these were illegal in most or all states. Instead, the kids are to be punished by having their futures decided for them, though again the point is minimized in the screenplay (note how Art's {de Wilde} one smile comes at the end, the required happy ending).
I'm not taking sides here, just trying to point out how a complex social issue is narrowed down to a single morally acceptable solution, typical of that strait-jacketed decade. Nonetheless and despite the loaded deck, I suspect the movie deals about as sensitively with the issue as conditions of the time would allow. However, canny viewers can learn a lot from this about the origins of the 1960's youth rebellions.