I'm not a teacher, so I don't understand which parts of education on biology/sex parents are allowed to exclude their children from.
But I think it's very concerning if parents are allowed to prevent girls having education on their own bodies as they approach puberty. Girl's rights to this information should override any sensitivities about 'culture' or 'religion'. No woman in this country should have her first period without understanding what is happening, or have her first experience of sexual intercourse without understanding how babies are made.
We shouldn't respect 'cultural taboos' if they damage women's health. All women should have a right to know that bleeding after sex is not normal, and should be investigated because it may be a sign of a cancer that could kill them.
Not all 'cultures' are equal, and we shouldn't allow any culture or religion to overrule women's rights in the UK.