Schools should not be allowed to promote marriage over homosexual partnerships, academics have said.
The UCU says homosexuals should not be treated as inferior
The University and College Union said any negative characterisations of homosexuality by teachers should be regarded as discrimination.
The UCU said new regulations to outlaw discrimination against individuals on grounds of sexual orientation did not go far enough.
The union said change would only come about as a result of education.
Delegates at UCU's annual conference in Bournemouth unanimously backed the motion which said: "All negative characterisations by teachers of lesbian/gay/bisexual/transgender people, identity and lifestyle should be outlawed and classified as an act of discrimination and an incitement to hatred based on sexual orientation."
Government guidance on new sexual orientation regulations says that faith schools will not be breaking the law if they continue to teach that homosexuality is a sin.
The guidance states: "If a faith school (or indeed any school) teaches that the Christian and Muslim faiths decree that same-sex sexual activity is a sin, then the school will not be acting unlawfully.
"Similarly, if a pupil asks a teacher his views on homosexuality and the teacher gives his view, then again, that teacher will not be acting unlawfully."
Alan Whitaker, a lecturer from Oxford and Cherwell Valley College in Oxfordshire, who put forward the proposal, criticised the new regulations.
He said the rules did nothing to stop "the negative characterisation of lesbian, gay, bisexual, transgender lifestyles by teachers".
"The regulations actually say that there is nothing to stop teachers proclaiming the superiority of heterosexual marriage.
"The regulations say it's unlawful to characterise same-sex relationships as inferior.
"But to my mind it's rather difficult to see how you can do the one without implying you are doing the other," Mr Whitaker said.
"Legislation cannot do all the work. It may remove the injustice but it cannot change attitudes.
"If attitudes are to change that will come about as a result of education.
"And that makes it vitally important that teachers do not instil negative images of same-sex relationships and transgender people in those that they teach."
Stephen Desmond, from Thames Valley University, told delegates: "We must never allow freedom of religion to be hijacked and used as a pretext to discriminate against gay and lesbian teenagers in schools."
A spokesman for the Department for Education and Skills said: "These regulations ensure that everyone is treated fairly and with respect, no matter what their sexual orientation, religion or belief.
"We trust teachers to use their professional judgement to decide which resources or publications can support teaching and learning in the classroom.
"Existing guidance makes clear that teaching should meet the needs of all young people whatever their family circumstances or developing sexuality and that all topics related to sex and sexuality are taught in a way that is age-appropriate."