The association called for better training and improved resources after a survey found four out of five teachers did not feel confident about the subject.
Sion Humphreys, from the NAHT's education management department, said: "The survey made fascinating reading as it highlighted that parents, teachers and governors all agree about the importance of SRE (sex and relationship education) for children.
"However, teachers have not been adequately trained to deliver it and have not had access to the range of resources they need.
"This is an urgent need that must be addressed if we are to be able to prepare young people for their future lives."
The survey, carried out by the NAHT, the National Governors Association (NGA) and the National Confederation of Parent Teacher Associations (NCPTA), found teachers, governors and parents all thought it important that schoolchildren in England were given lessons on sex and relationships.
Young people's embarrassment about asking questions was said to be the main barrier to providing effective sex education by 84 per cent of school leaders but a lack of training and confidence to address the subject came a close second.
Fewer than one in 10 (9 per cent) said they found current teaching materials "very useful" and more than a third (38 per cent) said either the school did not have materials, they did not how to access them or they simply were not useful.
Teachers suggested more DVDs and guest speakers would help them to improve lessons.
Fewer than one in 10 teachers and only 15 per cent of parents felt current SRE provision was preparing children "very well" for the future.
More than a quarter of teachers (27 per cent) felt it was preparing them "not well" or "not at all well".
More than eight out of 10 parents (84 per cent) said sex and relationship education should also be delivered at home but only six out of 10 (62 per cent) felt confident about broaching the subject.
David Butler, chief executive of NCPTA, said: "We owe it to children to get this right. If SRE lessons aren't preparing children properly for life as adults, then they need to be improved.
"More needs to be done to ensure teachers are trained in how to deliver sex education.
"It was particularly interesting to see that parents want to become more involved in this so that lessons could continue in the home.
"To do this effectively, parents need more information about what children are learning and specific resources to use with their children."
Gillian Allcroft, NGA policy manager, added: "It is vital that teachers receive adequate training if they are to deliver high quality sex and relationships education.
"Parents have an important role to play and they must be encouraged and supported to help with the delivery of SRE at home."
In response to the survey, Christine Blower, general secretary of the National Union of Teachers, said: "There is a strong argument for specialist training and CPD (continuing professional development) in sex and relationship education (SRE).
"Such CPD should cover how teachers can approach this important area of children's learning, having regard to cultural differences."
* The survey, carried out in partnership with Durex, questioned people online over a six-month period which ended in February this year through links from the websites of the NAHT, NCPTA and NGA. Respondents comprised 253 school leaders, 154 school governors and 1,003 parents of school-aged children.
Genevieve Edwards, director of communications at sexual health charity Terrence Higgins Trust, said: "It's vital that teachers and parents feel confident in talking to their students and kids about sex and relationships.
"We know from conversations we've had with young people that most of them aren't getting adequate information about sex from schools. In fact, eight in 10 say they have to turn to the internet to find out about sex, which isn't ideal as not all of that information will be accurate.
"If we're to bring down rates of sexually transmitted infections and teen pregnancies, young people need to be armed with the best possible information and advice to protect themselves.
"Teachers, parents, and young people themselves all have an equal responsibility to talk openly about sex."
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