“Mom, you won’t believe what my teacher’s talking about right now.”

That is the text message Regina Young, a parent in North Carolina’s Charlotte-Mecklenburg Schools, said she received from her seventh-grade daughter one day.

“Before I could even respond to her text,” Ms. Young said, “she told me how uncomfortable she was to hear her teacher tell stories about her transgender home.”

Ms. Young was just one parent who participated Monday in the Sex Ed Sit Out, an international demonstration to protest what rally organizers say is a campaign by pro-choice and gay rights organizations to force their ideologies on children through public education.

The movement began among parents in Charlotte, North Carolina, but spread to 16 cities in four countries in the months leading up to the event. According to the Sit Out’s website, rallies were also held in Calgary, Alberta; Mulgrave, Australia; Bloomington, Indiana; and London.

For the Sit Out, parents were asked to pull their children out of school for the day, accompanied by a note to the principal explaining the absence as a protest against “pornographic” sex education.

It’s the latest culture war to be fought on the grounds of America’s public schools, closely following national walkouts against guns and abortion.

On Monday, parents picked out two organizations for particular scorn: Planned Parenthood and the Human Rights Campaign.

The nation’s largest abortion provider and America’s leading LGBT rights advocacy group, respectively, have written sex education curricula that public and private schools across the nation have adopted.

Welcoming Schools, the pamphlet written by the Human Rights Campaign, is marketed to elementary school educators as a guide to prevent “bias-based bullying.”

Ms. Young said forcing radical sex education on children is a “form of bullying” itself.

“This is not about bullying,” she said. “We already have a policy against bullying. Why can a school teach and promote a sexual agenda to our children and violate our parental rights, giving permission to more of what’s happening and what happened to my daughter? It is not acceptable.”

At the demonstration in Charlotte, Tami Fitzgerald, executive director of the NC Values Coalition, said the rhetoric about bullying is a “Trojan horse” to give the gay rights movement “complete access to our kids in public schools.”

She said Ms. Young’s story is representative of what hundreds of other parents have told her.

“About a year ago this time, we began receiving phone calls from parents, teachers and counselors in CMS, Charlotte-Mecklenburg Schools,” Ms. Fitzgerald said. “They were saying, ‘Please help us. You cannot believe what they are teaching our children in public schools.’”

Welcoming Schools includes a list of children’s books that promote gay and transgender rights. Two of the books on the list, “Red: A Crayon’s Story,” about a blue crayon that identifies as a red crayon, and “Jacob’s New Dress,” about a boy who wants to wear a dress to school, were read to kindergarten students in Charlotte-Mecklenburg Schools, Ms. Fitzgerald said.

Protesters held up signs that read: “Let kids be kids,” “My child my choice” and “How dare you take funding to destroy my child’s innocence!”

Caryl Ayala, a former public school teacher, said about 15 people participated in the protest she organized in Austin, Texas.

“We are uniting with parents across the globe to demand that our rights as parents be respected regarding the teaching of sexuality and sexual orientation,” Ms. Ayala said.

Laura-Lynn Thompson attended a demonstration outside of the Parliament building in Victoria, British Columbia. She estimated the attendance at 100 to 150.

“We have had enough, and we’re not going to take it anymore,” Ms. Thompson said. “Canada is a nation that has a Charter of Rights, which include freedom of religion and freedom of conscience. Our rights have been trampled by the sex-ed curriculum in our schools.”

Heidi Pezdek, a member of Indiana’s Salt & Light chapter, helped organize the rally in Bloomington and estimated the number of participants at 40 to 50. She said walking into schools these days is like “walking into Sodom and Gomorrah. It’s all about sex.”

Leading social conservatives also backed the demonstrations.

The Rev. Franklin Graham said parents need to be aware of what public schools are teaching their children.

“Your children are in danger of being exposed to ‘Pornography 101’ under the guise of sex education in many schools,” Mr. Graham wrote in Facebook. “Today some parents across the country are pulling their students out in protest of the sexualized school curriculum being promoted by the progressive agenda. Know what is being taught in your child’s school and be prepared to walk out. I encourage you to be involved, know what’s going on, and let your voice be heard.”