Many of the nymphs do appear to have had breasts added later, in a darker pen, but if one takes an overall look at them, many of those already had a light-colored breast on the edge (not added on, but part of the original drawing), which makes it look like at least some of them were sketched in outline, with details left for later (whether by the same illustrator or someone else, I don't know). For those drawn from that angle, the gender is easier to determine. For those facing forward, it would be more difficult to know if it's male or female, but even so, often one can tell by looking at the belly, hips, and hair, which the illustrator has distinguished in most cases.
When I took a long look at the male-looking figures (with and without genitals), I got the impression the genitals were only included when they were directly relevant to the subject matter. For example, the page that appears to be a You are not allowed to view links.
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This further gives the impression that the nudity is not gratuitous, that it's used when it's appropriate, not just in bathing scenes and biological-looking sections, but in many of the pages that are possibly based on old myths (many old myths were of Pagan origin and the Pagan belief system considered nudity and procreation as normal rather than sinful and depicted nudity quite broadly).