It makes perfect sense. A discussion of sex should be holistic - that includes condoms, birth control and all the stuff that makes social conservatives so uncomfortable. Why? Because they exist and ignoring reality is never the right answer.
"Holistic"? Then you ought to include explicitly religious points of view, too, otherwise it's not really holistic.
But you can't do that in a public school. So it's already a given that you can't have a holistic discussion of sex, because explicitly religious values -- like mine -- aren't going to be permitted, except perhaps to be mocked and derided.
Given that my values and my value system are already written out of the picture, the least-objectionable position left to be taught is abstinence only.
I am, for the time being, blessed enough that I don't have to send my kids to government schools. I send them to schools which either explicitly teach my values, or which understand clearly that I pay the bills and my checks depend on my kids not being taught objectionable points of view.