Sex education classes are failing to include appropriate discussions of pornography, according to a survey, despite concern among experts about young people’s unfettered access to online porn sites.

A poll of 16- and 17-year-olds revealed “significant gaps” in relationship and sex education (RSE) in schools, according to the Sex Education Forum. More than half (52%) of those who took part in the survey said they thought more time should be spent on RSE.

More than a quarter said there was no teaching about pornography. Three out of 10 said they were taught nothing about sexual pleasure and two out of 10 did not receive teaching about signs of an abusive relationship.

The survey of 1,000 pupils in England, conducted on behalf of the Sex Education Forum and the National Education Union (NEU), also found that 23% of students were not taught how to recognise when someone was being groomed for sexual exploitation.

And among other worrying findings, 18% said they were told nothing about how to find help if they were sexually assaulted, and 23% were not taught how to tell if a particular relationship was healthy.

Other gaps in teaching included female genital mutilation, which was neglected for a third of pupils; 22% said they were not taught about LGBT+ issues; and 12% learned nothing about HIV.

Although 45% of pupils rated the quality of their RSE as good or very good, 34% said they wanted the subject of RSE to be treated more seriously.

Lucy Emmerson, the director of the Sex Education Forum, which works to promote high-quality RSE, said student satisfaction regarding sex education had improved over the last three years.

However, she said: “The confidence of schools to address important topics such as abuse, grooming, LGBT issues, pornography, sexual pleasure and FGM will be critical to ensuring that RSE meets the needs of children and young people growing up now, so they are safe and savvy as they mature into adults.”

Last year ministers promised to overhaul RSE and make it compulsory in schools in England for the first time, after sustained pressure from campaigners. This month, however, women’s groups said the government’s proposals, published for consultation, focused too heavily on self-restraint and were doomed to failure unless radically revised.

The End Violence Against Women Coalition (EVAWC), an umbrella group of more than 80 organisations, sent a letter to the education secretary, Damian Hinds, saying the guidance was “squeamish”, making only one mention of pornography and minimal references to menstruation.

Mary Bousted, a joint general secretary of the National Education Union, appealed to the government for more money to develop RSE. She said: “We know that schools’ budgets are stretched to breaking point and, in this climate, the professional development and support of teachers must vie with other demands on their school’s finances.

“Only with good training, guidance and support can teachers in all schools be ready to deliver high-quality RSE by 2020, and central government has to factor this into budget allocations for schools.”

Emmerson said: “We train hundreds of teachers each year and have seen growing awareness of the elements of high-quality RSE. We need to go further and accelerate progress in schools in the run-up to 2020, when the subject becomes statutory, so all schools can deliver on children’s right to information about their bodies, growing up, sex and relationships. Government must invest to achieve this.”