Personal, social and health education classes are due to become part of the compulsory national curriculum in primary and secondary schools from September 2011.
Family and faith groups yesterday condemned the proposals.
From the first year in primary school, four and five-year-old children will learn about different body parts, with lessons about sex from the age of nine.
At Key Stage Three (ages 11 to 14), pupils will learn about contraception, pregnancy, sexual activity and sexually transmitted diseases such as HIV.
Schools will also have to teach about ' different types of relationships, including those within families and between older and young people, boys and girls and people of the same sex, including civil partnerships'.
At Key Stage Four (ages 14-16), they will continue to learn about contraception, sexually transmitted diseases and same-sex relationships.
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Guidance, which is due to become statutory, states: 'Students should address the role and benefits of marriage and civil partnerships in stable relationships and family life.'
Schools will also have a new duty to give pupils a financial education, teaching them about subjects such as the credit crunch and how to handle their finances in later life.
In addition, they will have to cover the dangers of alcohol and drugs.
Simon Calvert, of the Christian Institute, said that 'pressing the virtues of homosexuality' could lead to more experimentation, which could be 'harmful' to children.
He said: 'What we don't want to see is vulnerable young people being exploited by outside groups which want to normalise homosexuality.
'If this guidance purports to force faith schools to teach things which go against their faith then it is profoundly illiberal and must be resisted at all costs.'
Norman Wells, of the Family Education Trust, said: 'Making PSHE a statutory part of the national curriculum could be used as a vehicle to promote positive images of homosexual relationships.
'It is difficult to see how teaching children as young as 11 about same-sex relationships and civil partnerships fits in with a study of personal wellbeing, and many parents will be very concerned about the prospect of such lessons being imposed over their heads.'
Under current rules, schoolchildren must be taught the biological facts of reproduction, which usually happens in science classes.
Every school must have a sex education policy, but there is no statutory requirement for teaching about relationships and the social and emotional side of sex.
The Government backed the move to a statutory PSHE curriculum last October and asked Sir Alasdair Macdonald, head of Morpeth School, in Tower Hamlets, East London, to report on its implementation.
His independent review yesterday recommended allowing parents to retain the right to withdraw their children from sex education classes.
Governing bodies will also be able to tailor the statutory curriculum around their ethos, for example Catholic faith schools could teach about contraception but say that its use runs contrary to their religious beliefs.
Schools Secretary Ed Balls said he would legislate to put the recommendations into effect, subject to a four-month consultation. He would keep the right of parents to withdraw their children under review.
He said: 'Compulsory PSHE will mean consistency and quality, so all children can benefit.'
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