I'm the mother of sons, no daughters, but I've just remembered those long long showers they always took

As to the original question about the double standard: the way i remember it, out parents implied that boys were uncontrollable randy creatures, and that was just nature. But girls, well, sex was something to be avoided at all costs when single, and endured when married. Girls who enjoyed it were bad or weak. If they had sex outside of marriage they were 'giving in' to the boy, and being incredibly stupid.
This double standard ensured that girls never discussed masturbation with their friends.