A report published on Wednesday claims that schools across England are using books that cover subjects such as orgasms and foreplay.
The study, by the Christian Institute, found a range of local councils recommended titles for use in primary sex education lessons that contained highly explicit references.
One book for children aged just five contains cartoon images of a couple having sex and another compares sex to skipping.
Another text for older pupils in primary schools and the first few years of secondary education features a card game in which pupils learn terms such as “anal intercourse” and “oral sex”.
The Christian Institute – a charity that promotes Biblical teaching – said parents were being kept in the dark about the content of resources used in sex education lessons.
The institute suggested a current Government review of the curriculum should give parents the legal right to veto the most "shocking sex education materials". Families already have the power to remove children from sex education altogether.
Mike Judge, the charity’s spokesman, said: “Most parents would be deeply upset if these materials were used with their primary-aged child.
“If public bodies believe these resources are suitable for young children, there is clearly a problem with their judgment and more control needs to be given to parents.
“Parents must have the right to be fully consulted about materials. They must be able to review them, and veto any that are unsuitable.”
Sex education is currently not compulsory for pupils aged five to 11, although all primary schools must have a policy on the subject. In most cases, schools choose to teach it as part of non-statutory personal, social, health and economic education (PSHE) classes.
In the latest study, "Too Much, Too Young", the Christian Institute investigated the content of sex education books recommended by local councils for teaching in primary schools in their area.
According to the institute, one text – “How Did I Begin” – is recommended for children aged five onwards in areas such as Devon and Wiltshire and features a cartoon sketch of a mother and father having sex, with the line: “As they cuddled, your dad’s penis moved slowly inside your mum’s vagina and the sperms flowed out.”
Another book – “Where Did I Come From?” – is recommended for pupils aged seven upwards. It tells pupils slang words for breasts and the joys of tickling boys’ and girls’ private parts, adding: “Making love is like skipping. You can do it all day long.”
A “Sex and Relationships Education” computer programme produced by BBC Active is recommended for use in primary schools in areas such as East Sussex, Worcestershire, Cornwall and Sheffield. It features a computer-generated image of an erect penis.
A book for boys – “What’s Happening to Me” – is recommended for pupils aged seven-plus in one area, and older children in other authorities. A section headed “The Messy Truth” instructs pupils about foreplay and masturbation, including the line: “Boys can make themselves ejaculate by rubbing their penis up and down.”
An education pack employed by a series of councils asks older primary pupils to match a series of sex education terms and definitions. This includes words such as “anal intercourse”, “bisexual”, “oral sex”, “prostitute” and “orgasm”.
Another book – “Let’s Talk About Sex” – is recommended for pupils 10 and over in some areas. According to the institute, it features a chapter on homosexuality, with the line: “As children are growing up, boys become curious about other boys and girls become curious about other girls. They may look and even touch each other’s bodies. This is a normal kind of exploring.”
A spokesman for the Department for Education insisted the Government did not recommend any teaching materials to schools, adding: “By law, schools must make sure that [sex and relationships education] classes are appropriate to pupils’ ages and maturity. It’s down to teachers themselves to use their professional judgement in deciding this – and its common sense to involve parents closely in this.
“Parents retain the right to pull their children out of any SRE class, outside statutory science, if they are unhappy with teaching.”