Government guidance on school sex and relationships education policies is out of date and is older than the majority of pupils learning about the subject, the Terrence Higgins Trust said.
The guidance for schools in England was written 16 years ago in 2000 - before the advent of social media, smartphones or equal marriage and Civil Partnerships, according to a new report by the charity.
The report also details the findings of a poll of 900 young people aged 16-24 which found the majority were not taught about consent in sex education - 75% said the topic was not covered.
Meanwhile, 95% said there was no mention on LGBT sex and relationships during their lessons.
Half of those surveyed rated the sex and relationships education (SRE) they received in school as either "poor" or "terrible".
The HIV and sexual health charity said where sex and relationships education is happening, it is usually limited to biological topics.
"In this report, we've seen the stark reality of SRE in this country and heard saddening stories of how one generation of young people have been exposed to low self-esteem, homophobia, bullying, unhealthy relationships and poor sexual health, as a result of the lack of quality SRE in our schools," said Terrence Higgins Trust chief executive Ian Green.
"Without trusted information from schools on anything other than the biological basics of heterosexual sex, young people will turn to less reliable sources such as the internet or their peers as they navigate life outside the classroom. We must end this silence and make SRE mandatory in all schools if we are to tackle this safeguarding crisis."
On the 16-year-old guidance, he added: "It is shocking that government guidance offered to schools on SRE is older than nearly all of the students themselves.
"Young people are getting information about sex and relationships in a world before social media existed, before smartphones, before equal marriage or Civil Partnerships.
"It is wholly unfit to prepare them for the realities of sex and relationships in 2016."
A Department for Education spokesman said: "High quality sex and relationship education is a vital part of preparing young people for life in modern Britain - helping them make informed choices, stay safe and learn to respect themselves and others.
"Our guidance is clear that young people, whatever their developing sexuality, need to feel that sex and relationships education is relevant to them and sensitive to their needs."
"We also expect all school to deliver PSHE to a high standard. We know that the vast majority of schools and teachers recognise the importance of PSHE, and trust teachers to tailor their lessons to best suit their pupils. We are focusing on raising the quality of PSHE teaching and working with leading headteachers and practitioners to look at how best to achieve this."
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