Compulsory SRE lessons would just pile on more pressure, say primary heads.
Calls for a new law making the teaching of sex and relationship education compulsory in Welsh primary schools were rejected this week, with heads saying the extra responsibility and workload would not be welcomed.
There were also criticisms that it was yet another example of teachers being asked to act like parents.
Chris Bryant, Labour MP for Rhondda, is behind the call for a new Wales-only measure, debated in Parliament on Tuesday.
He claims making SRE statutory for all primaries would help cut teenage pregnancy rates in Wales - currently the highest in Europe.
Assembly government guidance already recommends SRE should by taught at primary level, and many schools incorporate it within personal and social education.
But it is left to individual schools as to how far they go with informing pupils of the facts of life, as well as relationship issues.
Sue O'Halloran, head of Garth Primary School in Bridgend, said placing a legal requirement on schools would be an unnecessary burden. "SRE is taught at the school already but only in Year 7," she said. "I am happy with the guidance as it stands."
Ann Davies, head of Ysgol Y Dderi in Lampeter, said her school only taught pupils about puberty.
"I would like to know when we would have extra time to arrange this if it becomes law in primaries. Children seem to be pushed into adulthood so quickly these days."
But David Tyler, head of St Thomas Community Primary School in Swansea, said SRE had been taught in Y6 for 10 years. Only one parent had withdrawn their child from the lessons during that time, he said.
Latest Assembly government figures for 2005 reveal the conception rates of teenage girls aged 15-17 went down slightly in 2005 to just over 43 girls in every 1,000 (4.3 per cent) compared with 2004. But this was still higher than in England by just over two girls per 1,000.
All Welsh secondary schools have to include sex education as part of the national curriculum. Primaries can opt out under the Education Act 1996. Assembly government guidance, Sex and Relationships Education in Schools, recommends primary schools should teach SRE.
David Evans, NUT Cymru secretary, said a new law would "be another case of heads having to act as the parent."
Under his proposals, Mr Bryant said parents should have power to withdraw their child from SRE lessons if they were not happy. He claims teaching children of pre-secondary age about contraception and how to say no to sex could prevent pregnancies. He also said teachers should have extra training.
Mr Bryant added: "Media pressure is often towards early, and illegal, sexual experiences."
Leader, page 28.