Education is less expensive than ignorance.
The father believed that children should be kept innocent about sex until they were old enough to need to know. I don't particularly agree with his view, but I would have no problems with his action if the children were five or six. But the oldest child, a girl, was nine. And a girl who's old enough to menstruate is also old enough to become pregnant. And a girl who's old enough to menstruate is old enough to get pregnant.