A more basic question is this - why do schools believe it is their responsibility to teach anything about families at all? They’ve already got their hands full trying to teach the 3 R’s. Many teach history as some type of revisionist social studies, as opposed to just teaching history. IMO, public schools should stay away from teaching anything about families and family structure. That teaching should come from the family or the church.
I agree with you to the nth degree. In VA, parents are allowed to opt their children out of FLE (or family life education). Mine never attended once as I felt it was my job to decide when it was proper for them to hear about the 'birds and the bees.'