A 4-year-old girl from Denver, Colorado was kicked out from a preschool last month after her parents complained about books that were read to her class, including one about same-sex couples.

The mom, R.B. Sinclair, told the Denver Post that upon learning that her four-year-old daughter was being taught about homosexuality, she told the school administration that she wanted her daughter out of the class discussions.

However, Montview Community Preschool and Kindergarten in Denver told her that it was not possible and said the materials being taught were part of its anti-bias curriculum.

Anti-bias curriculum is part of a growing trend in public and private schools.

"Biases start as kids get older and start to see differences as negative. At a young age, kids are exploring all different kinds of things. It's about just providing them with all these experiences," said Kim Bloemen, director of early childhood education for the Boulder Valley School District.

In a letter, the school urges parents to inculcate "inclusive culture at home."

"Begin to Embrace the Unknown. Look for books that are very different from your own family and personal experience, but that you know are the experiences of other families in your school or neighbourhood or other circles of connection," the letter read.

Sinclair explained that her daughter, who is part of a Muslim and Western family, is too young to know the difference between anatomy and identity.

"I think at this age they don't know what bias is. They could have kids from Mars and they would still play with each other. It's not that she isn't exposed to diversity, because it is the world we live in, but how are they having these conversations?" she said.

Sinclair said one day, her daughter came home and said that her dad might no longer like girls. She added that her daughter's education is being interrupted by the focus on diversity.

Two days after meeting with the principal, Sinclair was given a letter by the school that stated that it was the girl's last day in school and the situation was "not a good fit."

"Meanwhile, there was no consideration for the bias against my family's culture, faith and concerns," Sinclair said.

The girl is now staying at home, the mom said.