The Government has accepted recommendations that sex and relationships education becomes compulsory from age five onwards.
The Christian Institute has joined other campaigners in warning that the plans will lead to the sexualisation of children and will undermine parents.
A BBC viewers’ poll coinciding with the announcement showed that three quarters of the 2,000 respondents said it should be up to parents to tell children about sex.
Less than a quarter thought the responsibility should lie with teachers.
The Government says that the lessons will come as part of a Personal Social and Health Education (PSHE) package that will be mandatory from ages 5 to 16 and will also cover drugs, alcohol and financial management.
Parents will be given some say in what is taught, ministers insist.
The minimum for primary schools will include “naming parts of the body” and “being able to talk about feelings and friendships.”
Last month the FPA (formerly known as the Family Planning Association) – one of the groups involved in recommending the new plans – launched a sex comic which asked children aged six and seven to identify correctly the vagina and testicles on a picture of a naked girl and boy.
Another sex education resource produced by Channel 4 Learning asked five-year-olds to point out the clitoris.
Commenting on the move, Labour MP Geraldine Smith said: “To start sex education at primary school is to rob young children of their innocence.”
Mike Judge, Head of Communications at The Christian Institute, commented: “Secondary schools already provide extensive sex education and it has utterly failed to improve teenage sexual health. Extending this to primary schools is a step too far.
“It will undermine parents as they face the difficult job of bringing up their children. The best people to teach children about sex and relationships are their parents.
“In a culture that is obsessed with sex, schools should be one place where children are allowed to get on with life without facing pressure to deal with things they aren’t ready for.”