Pupils should be taught a “positive portrayal” of same sex relationships in school sex education policies as a means of promoting equality, the National Union of Teachers resolved at its annual conference in Harrogate on Sunday.

Delegates overwhelmingly approved the motion on lesbian, gay, bisexual and transsexual (LGBT) rights, asking that “a future government must tackle the embedded homophobia, biphobia and transphobia that exists in schools and create a positive climate of understanding about sexuality and gender fit for the 21st century.

“This must include a commitment to make it easier to discuss ideas about sexuality and gender so that students and teachers are more confident to identify as LGBT and work in schools without fear of prejudice.”

Speakers told the conference that, despite recent gains, many schools still remained hostile places for lesbian and gay teachers and pupils, while same-sex marriage remained an uncomfortable subject in spite of its improved legal status.

The motion calls for extra training for teachers to help them deal with homophobic and transphobic incidents and for LGBT equality and rights to be included as part of initial teacher training courses.

The motion also called for an incoming government to “make it compulsory that all schools’ sex education policies include a positive portrayal of same-sex relationships”.

Christine Blower, the NUT’s general secretary, said: “We need education policy that develops curriculum for children and young people that supports the democratic values of a diverse Britain, including LGBT equality.

“Lesbians, gay men, bisexual and transgender people are all part of their local school communities as teachers, support staff, students, parents and governors.”

Sex and relationships education is mandatory for pupils at local authority maintained secondary schools in England, but is not compulsory within primary schools or in academies or free schools.

Last year, Ofsted began to include attitudes towards homophobia as part of its school inspection programme evaluating how well schools were preparing pupils for life in modern Britain.

A Church of England spokesperson said: “We have not studied the proposals in detail but we introduced new anti-homophobic bullying guidelines in all our schools last year and are fully committed to sex and relationship education, that allows room for exploration and discussion of relationships, within a framework of Christian values.”

Later, the union’s delegates debated a motion calling for a strike ballot to be held if a post-election government failed to improve school funding in England and Wales. But the conference ran out of time to vote on the motion following procedural delays, so it will be considered on Monday or Tuesday.

Paul McGarr, a teacher from east London, told delegates: “There’s a storm coming in education” over funding cuts likely to follow the spending plans of both Labour and Conservatives.

“If you make the wrong decision, we will fight whoever is in government,” he said.