Custardo
I agree with you that the government's solution to many problems seems to be to deal with it through schools, when more emphasis should be put on dealing with problems in wider society rather than a captive audience of school children. I think there has been a confusion between the role of parents and the role of schools.
But I do not see how schools can avoid teaching some SRE. For 5 year olds, SRE is not about sex; it is about relationships. If your child attends school, they are having a a relationship with other children and the teacher, and the school has to talk about relationships, and what is appropriate in school and how these institutional relationships are different to home relationships. It is unfair on other children if your child has not been taught how to understand this, and only school can teach this because home is not an institutional environment.
When your children are older, schools have to teach the mechanics of reproductive sex, because it is basic fact of biology and so has to be taught and tested if children are to leave school with a qualification in science.
Schools also have to teach some elements of morality (not sexually harrassing people, not saying gay student are disgusting) because to premit some forms of sexually related behaviour is illegal both in and outside of the school environment, and such behaviour is common in schools and has to be dealt with by the school.
The reason that SRE is going to tie these disparate concepts together is that secondary school children have requested it.