They want to make "age-appropriate" lessons compulsory in all state-funded schools, as part of a "curriculum of life", which would also include lessons on managing money and citizenship.
Their proposals will be included in the party's manifesto at the next general election in 2015.
The idea has been backed by Labour, although the party accused the Lib Dems of failing to deliver it so far.
However, the Conservatives said they felt teachers knew best how to deliver such education, as opposed to politicians.
Christians have been sharing their thoughts on whether they would support the idea of compulsory sex education in schools from the age of seven.
Premier listener Kate is concerned it could confuse some children that young: "I think at seven, they should be concentrating on the 'three Rs' and teaching them to behave and love each other in a polite, nice way."
Donald, who has experience working with young people at Christian schools, says his main concern is about how the lessons would be run and what would be in them: "What would the content of that education be? And who would be allowed to put together that content, in a democratic way?"
However, Premier listener Joyce said she would support the idea.
She argued that children are exposed to so much explicit content on television, film and the Internet that many of them already know the facts of life before the age of seven.
She's concerned that if they're not taught by professionals in schools, they could pick up misconstrued ideas about sex from the media.
Click below to hear what other Premier listeners had to say about the proposals: