And the vast majority - 95 per cent - were not taught about lesbian, gay, bisexual and transgender (LGBT) relationships at school.
The survey of 900 people between the ages of 16 and 24 found that one in seven had not received any sex and relationships education (SRE) during their time at school. Almost two-thirds - 61 per cent - had been given SRE lessons a maximum of once a year.
Half of those who had received SRE said that the lessons were poor or terrible. Only two per cent rated it as excellent, and 10 per cent as good.
In addition, 75 per cent said that they had not been taught about issues of consent during sex, and 89 per cent had not been taught about pleasure in relation to sex.
Only 5 per cent had been taught about LGBT sex relationships. And only 3 per cent had received any kind of education about gender identity.
Meanwhile, 59 per cent said that they had not been given any information about HIV while at school, or did not remember being given any information about HIV.
Biological basics
SRE is now mandatory in state-maintained secondary schools. However, this means that academies, free schools, private schools and primaries are under no obligation to provide it. Earlier this year, the government refused to make SRE statutory in all schools, against the advice of the Commons Education Select Committee, and many teachers and parents.
Ninety-nine per cent of the people questioned for the survey said that they thought SRE should be taught in all schools. And 97 per cent believed that it should include LGBT relationships.
Ian Green, chief executive of the Terrence Higgins Trust, which commissioned the survey, said: “Young people have been exposed to low self-esteem, homophobia, bullying, unhealthy relationships and poor sexual health, as a result of the lack of quality SRE in our schools.
“The government’s quiet blocking of compulsory SRE will condemn another generation of young people to leave school armed with little to no information on anything other than the biological basics of heterosexual sex.
“We must end this silence and make SRE mandatory in all schools, if we are to tackle this safeguarding crisis.”
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