Over fifty MPs have written to the education secretary, urging the government to give stronger backing to schools on teaching about same-sex relationships, Newsnight has learned.
The move follows a row at a school in Birmingham, where a parent-led protest against such teaching forced the school to close early.
Damien Hinds has previously spoken in support of the school's teachers, who want to continue the lessons, but the group of MPs wants him to go further,
They write: "We ask you to provide absolute clarity in regard to relationships education in primary schools, which must be inclusive of all protected characteristics, and treat the different types of relationships in our society equally."
The MPs say it is "unacceptable" that a school had to close early - and seek a High Court injunction and exclusion zone - "due to nationally co-ordinated protests."
The letter welcomes Mr Hinds' former statement which encouraged schools to discuss "different" types of families, but says it "does not go far enough".
"We urge you to make it clear that schools have an obligation to teach about same-sex relationships in primary schools," the MPs say to Mr Hinds, adding that relationship education is "not a choice... It is a legal requirement."
The letter was written by Labour MPs Emma Hardy and Jack Dromey, and co-signed by MPs from across the political spectrum. Some members of the House of Lords also added their names.
Referring to the protests at Anderton Park school, the MPs blame "misinformation over the content" of the LGBT teaching, as well as "a belief that it is individual head teachers making choices to teach such content" for fuelling the disagreement.
Protests have been held outside the school by parents and others, arguing children are too young to learn about diverse families through reading storybooks. Some of the protestors say the lessons contradict their Muslim values.
The controversy has had an effect on both local and Westminster politics. The school's local MP, Labour's Roger Godsiff was given a warning by the party's chief whip on 14 June not to repeat his support for the protesters. He had said parents protesting against teachings about diverse relationships "have done nothing wrong".
Other schools have also faced anger over the lessons, which use books from the No Outsiders series to introduce children to transgender characters and same-sex relationships. Some parents have removed their children from school and head teachers have been threatened.
Education Secretary Damian Hinds issued this statement in response:
"At the heart of preparing children for life in modern Britain is making sure that they understand the world they are growing up in. It is a world that is different from 20 years ago, when this guidance was last updated, and this is a significant step that will help young people to look after themselves and each other - although the disagreements which we have seen do not centre on the new relationships curriculum, which has not in fact yet been introduced.
"A wide range of views were expressed during the public consultation, and I believe the guidance strikes the right balance. Our new guidance is clear that children should leave school having learnt about LGBT relationships, and I strongly encourage primary schools to teach about different types of family, including families with same-sex parents.
"I have been clear that protesting outside schools is unacceptable. No child should have to walk past a protest to attend school; neither should any teacher. We want dialogue and consultation with parents, but that does not constitute a veto; I support headteachers to make decisions about the curriculum, including ahead of the introduction of these new subjects."