It is certainly a sensitive, therefore important topic, which can be approached in a cross-curricular, integrative approach. It has to do with science, social aspects, ethics and more.
It is about knowing about health and our body (learn about the reproductive system, in the same ways, as we learn about e.g. muscle system, circulation system or respiratory system).
It is about learning about social behaviour (our rights, not to offend others, as we shouldn't do in anyway).
What is legal or acceptable, depends on the general context too. There are elementary school teachers in some countries who are afraid to teach relevant subjects. This fear is not necessarily because of a law or religion, but lack of social open mindness.