Thomas Pascoe from Coalition for Marriage and the former Bishop of Rochester Rt Rev Michael Nazir-Ali are among two dozen people to sign an open letter in The Times on Tuesday.
Referring to the proposed relationships education lessons, they claim: "The Education Secretary has made no mention of ensuring that children are taught about the well-established benefits associated with being brought up by married natural parents."
In March 2017, Justine Greening (pictured below) set out plans for primary school pupils to be taught about relationships, while secondary school students would take sex and relationships lessons.
She said at the time: "At the moment, too many young people feel they don't have the relationships and sex education (RSE) they need to stay safe and navigate becoming an adult.
"We need high-quality, age-appropriate content that relates to the modern world, addressing issues like cyber bullying, 'sexting' and internet safety."
Signatories to Tuesday's letter expressed concern "very young children" will be introduced to concepts such as homosexuality and transgenderism "at an age where these cannot be critically assessed".
They urge that a full public consultation into the proposals later this year "puts the rights of parents ahead of the power of the state".
They add: "Parents and guardians must retain the fundamental freedom to bring up their children in accordance with their beliefs and values".
Pascoe told Premier: "If you're going to teach adult education or adult relationships to children, it's very important to emphasise marriage.
"What's been astonishing is that in the initial guidance issued by the Depart for Education there's been no mention at all of traditional marriage, the cornerstone of our civilization.
"We believe that parents must retain the fundamental freedom that they've always have to bring up their children in accordance with their own beliefs and values without the state intervening and trying to push a certain view of alternative lifestyles to children."
A Department for Education spokesperson told Premier: "We want to help all schools deliver high-quality Relationships Education so that young people are equipped to have healthy and respectful relationships.
"We have set out clearly that the subject will be age-appropriate and will focus on preparing pupils for life in modern Britain, and ensuring they understand the laws of the country on relationships.
"We have also been clear that parents should be involved in the development of these subjects and will set out requirements and expectations in the regulations and guidance."