Sex education is failing millions of young people, with most students turning to porn to fill in gaps in their understanding, according to the National Union of Students (NUS).
Two-thirds of students questioned in a survey by the NUS rated their sex and relationships education (SRE) as "fair, poor or terrible".
Gaps in teaching were uncovered, with three-quarters (75%) saying they found out about sex and relationships through friends.
According to respondents, consent was never touched upon in lessons for two-thirds of them, while relationships were covered for less than half.
The NUS said more than a third did not rate their SRE positively on equality and diversity, with less than a fifth saying they were taught about LGBT (lesbian, gay, bisexual and transgender) relationships.
More than half felt the issues they needed to know about were not taught, with only a third feeling they could practically apply their SRE to their real life.
Students agreed that porn was now a standard part of a young person's life, but three-quarters felt it provided "unrealistic expectations".
The NUS has now called for political parties to commit to statutory SRE in their general election manifestos that goes beyond "simple biology".
NUS vice president Colum McGuire said: "Sex is not a science lesson. People are being left with gaps in their education. Inadequate SRE puts young people at risk."
He added: "SRE is failing millions. NUS runs consent workshops on campuses across the country, 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."
Jack Wallington, director of community at the Student Room, said the NUS's findings mirrored many comments they hear from young people.
He said: "Students regularly report a general lack of practical advice and, worryingly, inadequate or incorrect information about sexuality, contraception and sexually transmitted infections (STIs), leaving a black-hole of questions filled by friends and websites.
"We'd like to see greater standardisation in our sex and relationships education programmes, which directly address the needs of young people instead of leaving them to hunt for pieces of the puzzle themselves."
:: A total of 2,502 students throughout the UK were questioned in an online survey.
A Department for Education spokesman said: "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.
"We also expect academies and free schools to deliver relationship education as part of their provision of a broad and balanced curriculum.
"We have set up a new expert subject group on personal, social and health education (PSHE) to support teachers, made up of leading professionals in the field, and will clarify the key areas on which teachers most need further support and produce new resources where necessary."