Thousands of children rang with pregnancy worries, the charity said
Advice on emotions and relationships should be made a compulsory part of sex education in schools, the children's charity NSPCC says.
It said nearly 50 children a day were calling its Childline helpline because they felt under pressure to have sex.
They also lacked basic knowledge about relationships, pregnancy and puberty, according to the charity.
Schools minister Jim Knight announced a government review in February of how sex education is taught in England.
As the law stands, schools in England only have to teach the biological aspects of sex.
The charity said that too many children were getting involved in sexual situations they could not handle.
Between April 1, 2007 and March 31 this year, 5,985 girls and 503 boys - including children as young as 12 - called Childline to discuss worries about pregnancy.
"Children are in the dark about the biological and emotional aspects of sex," said Sue Minto from ChildLine.
"This leaves them without the knowledge and skills they need to make informed, responsible decisions about their actions, and so they are getting caught up in sexual situations that could have serious implications in their lives."
She said children needed information about peer pressure, relationships and love, as well as help developing skills to make the right decisions about sex.