Why can't the teachers just teach sex ed?
Teachers are specialists in their subjects. They spend years becoming knowledgeable in their fields, but feel unqualified to teach sex-ed.
I'm sure biology teachers feel qualified to teach the functional parts, but no better informed than anyone else to teach about relationships, consent, sexuality, etc.
When I taught we were basically given a few lesson plans at the beginning of each term and left to pass it on to our tutor groups. Some teachers hated it, most thought it was a distraction from their actual jobs, some didn’t mind too much if they had a good relationship with their group.
I’d guess that most secondary schools would be big enough now to employ a specialist sex-ed teacher or two, but would anyone really want that job? And what sort of person might apply for a position like that?