Children find sex education "too biological" and schools should teach more about emotion, a report says.
Schools are only required to teach the biological facts about sex
The Sex Education Forum said personal social and health education (PSHE), which includes sex and relationships education, should be made compulsory.
Although schools in England have to teach the biological aspects of sex, PSHE is not a statutory requirement.
The Department for Education said sex education was expected to be taught in the context of loving relationships.
ChildLine has also called for better sex and relationships education.
Analysis of more than 5,800 calls to the NSPCC-run helpline highlighted a lack of awareness among young people, with ignorance and embarrassment about sex putting their health and well-being at risk.
ChildLine said it was calling on the government to review PSHE and make this a statutory requirement in England.
The Sex Education Forum, a network of organisations under the umbrella of the National Children's Bureau, commissioned GfK NOP to carry out a survey of 1,307 parents of young children.
They found 83% thought schools should teach about the emotional aspects of sex and relationships as well as the biological facts.
Co-ordinator of the forum, Anna Martinez, said: "There is a clear groundswell of support for making PSHE compulsory within schools."
Learning about sexuality, relationships, choice, delay, safer sex, risks and pregnancy choices are recommended to be taught within PSHE and are outlined in government guidance, but are not a statutory requirement.
The Forum claims PSHE is under-resourced and is delivered in many schools by non-specialist or poorly prepared teachers.
A spokesman for the Department for Education said: "We expect sex education to be taught in a context of caring and loving relationships in order to provide children with the information needed to prevent early sexual activity and unplanned pregnancies.
"We do not recommend specific resources for use in sex and relationship education.
"We trust teachers to use their professional judgement to decide which organisations can support teaching and learning in the classroom and which resources best support schools' sex and relationship programmes."