My daughter found one of those in a local bookshop when she was slightly "too young", i.e. about 7. I tried and failed to deflect her by pointing out other books on her traditional favourite subjects, to no avail.
The book I read didn't present it in a sexual way, the pictures were labeled so that a medical student can identify the different parts.
The book my daughter discovered (and I bought) was solely sexual.
Example: one diagram was a cross-section through a copulating couple.
Frankly that sounds like a parenting failure to me. The child is 7, the fact that you tried and failed to redirect her doesn't make much sense to me, you are the parent, you are the law, all you have to do is say "no, that's not appropriate for you at this age" and that should be that, case closed, there is no negotiating.
Oh riiiight.
Teach her not to be curious.
Teach her not to find answers by reading.
Teach her to deviously hide things from me.
Teach her that sex is a disgusting subject.
Teach her to be ashamed of asking questions and having independent thoughts.
Teach her that I'm not interested in what she is thinking.
Teach her that "might is right" rather than "right because reason X".
Good parenting... Not!
It was a good book. I was delighted she had found it and was fascinated by the subject. That's why I bought it for her so she could continue reading it at her leisure.
It paid dividends. Three years later she wanted chickens as pets. I didn't, and raised all sorts of objections. She went and did the research online, and countered every one of my objections with solid arguments. That delighted me, and I agreed we could have them. She specified what was needed in a coop and enclosure, and we built one together. Over the next six years she used her chickens as a spring board for learning all sorts of skills that we never anticipated.
More recently she has used those attitudes and skills to successfully start her own business. She's even been invited to demonstrate to others that how it is possible to start their own business.
But I suppose you think that, because I didn't want something, I should have been "a good parent" by stomping on her interests and ambitions.
If letting her read that book was a parenting failure, IMNSHO the world would be a better place if more parents failed.