It's seems pretty easy to understand some diseases can be transmitted, pregnancy exists, and consent is important at ages where you still wouldn't necessarily be teaching about mitochondria. It's embarrassing that even knowing basic names of anatomy and that different people have different anatomy is something that some kids still don't know by like pre-K.

The hardest parts is just the awkwardness of those topics with today's culture. I don't remember how old I was, but I remember my mom trying to teach about STDs and pregnancy, and my response was like "just don't have sex" and her reaction seemed to indicate to me that she disagreed but also didn't feel comfortable actually saying anything positive about sex and just assumed I'd change my mind once I reached puberty (which was probably not long after). But not being willing to talk about the positive aspects means teaching that these topics are taboo and leads to children being unwilling to talk to parents when they should be (even if just to ask for things like condoms).