This seems to have happened in the late 1930s or the early 1940s (he says "while I was working on the bomb".) From what I've heard, the US was very conservative at that time; girls & women in general did not have sex before marriage. And additionally was contraception widely available at that time? The sexual revolution happened in the 60s and 70s. So, this does not make any sense. Can someone enlighten me?
That's just good old day syndrome. I've had some very interesting talks with older folks about this very subject. They all agree that the prevalence of sex in the culture is certainly a bit more front and center today. However, men have always been men and women have always been women. Teen birth rates where at their highest in 40's, 50's and 60's after all. Contraception certainly has played a role in reducing that,…
I don't doubt the truth of this story (though there is certainly hyperbole); rather I find it hard to believe that 'sleep with' in this context actually means 'sex'. The main flags are A) Feynman is discussing with other people about it (afaik not acceptable socially then) and B) a sexual one-night stand would be even less so.
Finally I wager that the teen birth rate drop is solely due to an increase in education and contraption (and even legalized abortions). Obviously, there were some teenagers having sex in the 1940s (with a much higher liklihood of becoming pregnant right now), but, due to the risks and less acceptance than now, the rate was lower.