Children in schools in England and Wales could be taught about same-sex marriage should proposals to allow gay people to wed go ahead, according to internal Government documents released on Monday.
Measures introduced in 1996 require sex education lessons to include teaching about "the nature of marriage and its importance for family life", and the emails suggest that teaching material would have to be rewritten to cover same-sex marriage if the law changes.
The Department for Education (DfE) said that schools will continue to be under an obligation to avoid teaching anything which is deemed "inappropriate" to a pupil's age, religion or cultural background.
However, the Campaign for Marriage claimed that schools could be forced to teach children about gay marriage against parents' wishes.
The emails were obtained from the Home Office under freedom of information legislation by C4M, which is campaigning against gay marriage during the Government's consultation on a possible change in the law to allow equal civil marriage.
C4M campaign director Colin Hart said: "Marriage appears more than 3,000 times in law, affecting every aspect of our lives.
"It is simply impossible to redefine it without many serious unintended consequences, not least forcing schools to teach children about gay marriage, even if this goes against the wishes of the parents, children and teachers."
A DfE spokesman said: "The consultation on equal civil marriage changes nothing about teaching in schools.
"Every school has a requirement to ensure it does not teach anything that would be considered inappropriate to a pupil's age, religious or cultural background. This will not change."