It's very simple: to them straight relationships/people ought not be in a privileged position over gay/trans ones. Therefore if you can talk casually about a straight couple being married and having a family, you need to be able to casually talk about gay and trans relationships. If you can casually talk about hetero sex (which they have been pushing since at least the early 90s: Bill Clinton's Surgeon General was fired for pushing teaching grade school children to masturbate at some UN conference), then you must be able to talk about hetero sex.

What the Christians were saying in the 80s and early 90s has proven 100% correct: it would not stop with "teaching kids the basics" and would indeed be used to push all sorts of agendas on children contrary to the values and wishes of their parents. That promise being broken, why should the right uphold its end of the bargain by allowing talk about sex or relationships in schools at all? Not like 4th graders are in any better position to understand any of this. Not like there are fewer degenerates teaching grades 4+.