Well, not all parents are comfortable with such topics, and I do not think having children be completely ignorant about puberty is NOT the answer to the definite issues and problems of “comprehensive sex ed”, many get very scared and anxious about what is happening, and of course kids talk amongst themselves.
I respect parents who choose to opt out of sex education in public (or private) school, but just by being in school with other kids who didn’t opt out, their kids are going to find out what was in it, and this may not even be accurate but filtered through other kids. Even homeschooled kids can get exposed to other kids with odd notions. No way to prevent that unless the parent personally watches over every single interaction a child has with another.
AFAIK not even the Duggars do that, though they do closely supervise “courtships” it seems excursions with same-sex friends are not. And sometimes they depend on the younger kids to “chaperone” the teenage and up older kids who are courting. While I find this quite inappropriate, I also suspect they wouldn’t actually do that, without those adults with video cameras also following them around.
I do think this is quite reasonable, and if the school refuses your request, I’d not blame you at all for opting out.