The way this particular textbook explains female sexuality, the human race would come to an end if men and boys were to start keeping their pants on in the presence of little girls -- but since this is unlikely to happen, there's no need to worry: the life-affirming penis envy will continue. As to why girls are so envious, the book offers this explanation: "with this organ [males] can perform feats -- urinating standing up, etc. -- feats of which little boys are inordinately proud and which they exhibit to their girl friends on every possible occasion and of which she is incapable." Come on, O. Spurgeon English, M.D., don't leave me hanging with that "etc."! What else can little boys do with their weenises? Can they spin plates or something? Because while peeing standing up is a neat trick, I demand more before I will concede that the discovery that she lacks a penis devastates the little girl into a heterosexuality from which she never recovers.
Also, I don't know what kind of upbringing these researchers had, but personally I don't remember a lot of parading of genitalia in my childhood. Maybe the eighties were a lot more pants-on than the thirties.