Schools across England have received letters opposing the teaching of relationships and sex education (RSE) and LGBT equality, the BBC has learned.
Protests started in Birmingham and letters, predominantly from conservative Muslims, have been sent to a number of schools elsewhere.
One campaigner said relationship lessons due to start in schools in 2020 "proselytise a homosexual way of life".
Supporters of the lessons said there was a "lot of misinformation".
Letters opposing the lessons have been sent to schools in Birmingham, Bradford, Bristol, Croydon, Ealing, Manchester, Northampton and Nottingham, BBC Newsnight has discovered.
Some have also been sent from Christian parents in Kent.
Mainly Muslim families have been protesting outside Anderton Park Primary School in Birmingham after pupils were given books featuring transgender children and gay families.
Protest leader Shakeel Afsar: "All we are concerned [about] is we are having our children come home with material that contradicts our moral values."
Another protester, Amir Ahmed, said: "It's not about gay lesbian rights and equality. This is purely about proselytising a homosexual way of life to children."
When asked if he believed children could be "recruited to be gay", Mr Ahmed said: "You can condition them to accept this as being a normal way of life and it makes the children more promiscuous as they grow older."
He added: "Whether they become gay or not, they can still enter into gay relationships.
"They want to convert you, they want to convert your morality and that's just wrong."
Anderton Park headteacher Sarah Hewitt-Clarkson said she had spoken with parents, but days later leaflets circulated accusing the school of "lying" and having a "gay ethos".
Protesters have insisted they are not homophobic, but the BBC has seen Whatsapp groups with large numbers of contributors in which some people use homophobic language.
Labour MP Wes Streeting, who is openly gay and co-chair of the All Party Parliamentary Group on British Muslims, said: "When you are standing alongside people talking about the proselytising of children, a homosexual agenda, promiscuity, I'm afraid you're homophobic."
Shaykh Ibrahim Mogra, an imam from Leicester, said he was "disappointed" with the reaction of the protesters.
He said: "We really need to calm down and think very carefully, it is a very very sensitive topic."
Shaykh Mogra said Mr Ahmed's use of the word "proselytising" was very unfortunate, adding: "I don't believe there is an active effort on the part of LGBT communities to try and convert me and others to become gay people. It's something you don't choose into or opt out of.
"There is a lot of misinformation. It is not about promoting [homosexuality], it's about making our children aware.
"The whole driver for this is not the promotion of the LGBT agenda, it's about inclusivity and to ensure the bullying of such communities is ended."
Aaliyah Hussain, of Women Empowered against Racism, Injustice, Sexism and Extremism, said she had come across concerns in Bristol from parents and schools.
She said some children have been withdrawn from school and there are threats to remove others.
Ms Hussain said the "anxiety" is caused by the spread of information on social media, but it was "absolutely not the case" that this was division between religion and equality.
She said: "It's very dangerous to go down that road.
"This idea that it is religion versus equality is a misnomer because Muslims believe in equality and freedom as well."
The Department for Education said: "Pupils should be taught about the society in which they are growing up.
"These subjects are designed to foster respect for others and for difference, and educate pupils about healthy relationships."
Compulsory relationship and sex education lessons are due to start in all secondary schools in England from September 2020.