The public have a worrying lack of knowledge and understanding about sex, a survey suggests.
Adults displayed a lack of knowledge about sex, the survey found
A poll of 495 people by the Family Planning Association found some thought exercise or urinating after intercourse could prevent pregnancy.
The FPA said better sex education was needed, with researchers at Coventry University calling for lessons to be tailored to pupils' sexual experience.
The government said it was up to teachers to decide on sex education.
All secondary schools are required to include sex education as part of their teaching programme, but there are no rules over the content, leading to complaints that lessons are too biological.
The FPA survey, published to coincide with the start of Contraceptive Awareness Week, found that half of people did not know when was the most fertile point of a woman's menstrual cycle.
A third thought exercise, douching or urinating after sex could stop fertilisation and 89% did not realise sperm could live inside a woman's body for up to seven days.
On sex education, only 4% said their experience at school was excellent, 38% described it as poor and 18% said they did not get any.
Anne Weyman, chief executive of the FPA, said: "This survey exposes how far the current system of providing sex education is failing and also that people are acutely aware that it is letting them down.
"Reproductive biology is the only statutory part of the national curriculum and even this isn't achieving acceptable standards."
The FPA also said that better access to sexual health clinics could help improve adults' knowledge.
Coventry University researchers agreed sex education standards were not good enough, pointing out that different pupils had varying degrees of sexual experience and needed different teaching.
In a survey of 3,800 pupils aged 13 to 16, they found a quarter were sexually active, with nearly half of those not using contraception every time.
The team suggested teachers could use computer programmes to offer tailored education.
Lead researcher Professor Louise Wallace said: "Much of the teaching in schools is too biologically based."
Pupils needed to be given "sex and relationship education that equips them with the knowledge and skills to inform their choices".
The Department for Education and Skills said all secondary schools should provide sex education.
But a spokeswoman said it was up to teachers to use their "professional judgement" to decide what to teach in the classroom.
She added: "All parents have a statutory right to withdraw their children from all or part of sex education lessons, but not from statutory science lessons."