More than half of university students use porn to find out about sex, the National Union of Students (NUS) says.
A survey of more than 2500 students found that two thirds received no information about consent in their sex and relationship (SRE) lessons at school.
The NUS is calling for adequate SRE lessons to be a legal requirement.
The Department for Education (DfE) says they have set up a "new expert subject group" to advise schools on SRE.
A DfE spokesperson says: "Good quality relationship education is an important part of preparing young people for life in modern Britain, and our statutory guidance makes clear that it must be taught in an age appropriate way.
"Sex and relationship education is compulsory in all maintained secondary schools and many primary schools also teach it in an age appropriate manner."
Academies and free schools in England are also expected to deliver relationship education, the DfE states.
Of the 60% of the students who use porn to improve their knowledge of sex, three quarters said it gave unrealistic expectations.
A fifth of students claimed their SRE lessons made no mention of LGBT issues and more than a third felt their SRE did not rate positively on equality and diversity.
"SRE is failing millions," says NUS Vice President Colum McGuire.
"NUS runs consent workshops on several campuses, covering a vital aspect of SRE that this government misses.
"The current system almost completely ignores LGBT relationships. In a country where we passed an equal marriage bill, this is the height of hypocrisy."
As part of their New Deal general election manifesto, the NUS is calling for the winner of May's vote to make sex education a statutory requirement of schools.
MPs on the Education Select Committee are currently investigating SRE in schools and is due to publish its findings soon.
Both Labour and the Liberal Democrats say they will introduce compulsory SRE. The Conservatives have yet to reveal their plans.
In November, Newsbeat spoke with Luke Alexander, a 19-year-old who is HIV-positive. He's been campaigning for better education around sexually transmitted infections.
"While I admit that I did know about HIV and my infection is down to my own idiotic neglect of safe sex, it really is a shame that I didn't know enough," he said.
"Last year the schools watchdog, Ofsted, claimed more than a third of schools in England were failing to provide pupils with age-appropriate sex and relationships education.
"In my opinion, this continuing problem with sex education is stopping young people from learning anything about how to protect their sexual health and leaving an unlucky number to be afflicted by an incurable disease they haven't really heard of."