It’s about more than kids learning that sex happens, although that’s part of it. I’ve heard completely sane (American) parents stumble through explanations about why their children should never have to learn what sex is.
But more than that, it’s about power. This parent is mad that her child’s teacher is unmarried and pregnant because it’ll be harder to tell her daughter that getting pregnant without a husband will ruin her life.
It’s not the pregnant part that’s important, it’s the unmarried part. And lots of parents think they have control over their teachers’ personal lives mainly because they want control over their children’s genitals.