Same-sex marriage poses a "real threat" to the freedoms of teachers with traditionalist views, a Tory MP has warned.
David Burrowes said classrooms would be subjected to a "new state orthodoxy" where teachers who opposed gay weddings would be gagged for fear of being sacked.
His comments came as the former Archbishop of Canterbury Lord Carey said legal advice showed churches which refused to marry gay couples could be sued and teachers sacked for refusing to teach about them.
However Burrowes also lashed out at a pro-life group's "unhelpful and offensive" warnings that same-sex marriage could "gayify" straight children, insisting the issue was about marriage and not homosexuality.
The Society for the Protection of Unborn Children (SPUC) has written to secondary school heads calling for them to campaign against gay marriage.
As well as joining the warnings that "conscientious objectors" faced the sack, it highlighted fears schools would be forced to "promote homosexuality".
At a launch event in London, one platform speaker warned of "invidious sexualisation" and gave graphic accounts of what she said was unsuitable material already shown to young people.
"Lads who would otherwise not be heading into 'gayland' will go there," said Lisa Nolland, described as the convener of the Marriage, Sex and Culture Group of Anglican Mainstream.
Expanding gay sex education "will gayify our straight kids", she added.
The approach drew a stinging retort from Burrowes during a question and answer session. He said he found it "offensive that the debate gets into this area".
"That's not needed or wanted," he said, urging the group not to use the campaign against gay marriage for a more general attack on homosexuality.
He told the Huffington Post UK: "The campaign is about marriage, it is not about the issue of homosexuality. The focus is on whether marriage should be redefined or not."
"I didn't think it was helpful, or appropriate, or right, or relevant."
"I was invited to a meeting about the real concerns there are among teachers about gay marriage.
"The issue is about teachers in schools who want to uphold the view of marriage as between a man and a woman who will feel reluctant to say that's their view," he said.
"People will feel circumscribed by a new state orthodoxy.
"As MPs we can say it until we are blue in the face but out in the classroom there will be a real threat to their freedoms."
In his letter to peers Lord Carey said the advice of a senior lawyer prepared for the Coalition for Marriage campaign undermined the government's "quadruple lock" to protect churches.
Ministers intend to legally bar the Church of England from carrying out same-sex marriages and put other measures in place to ensure no church is forced to against its wishes.
But the advice from Aidan O'Neill from Matrix Chambers suggested the ban was "eminently challengeable" in the European Court of Human Rights.
Lord Carey wrote: "More worrying still has been the failure of the government to see that they are creating significant problems for those working in the public sector, for parents and even children.
"Like you I was horrified at the treatment of Adrian Smith, the housing manager from Trafford, who was demoted and had his pay docked by 40% for posting a message to his friends on a social networking site, saying that gay marriage in churches was 'an equality too far'.
"If the government presses ahead with its plans, cases like Mr Smith's will almost certainly become commonplace."
Colin Hart, campaign director of the Coalition for Marriage, said: "This detailed legal opinion puts a sword to the lie that the government's safeguards will not be tested in the European Courts and the threats to them are negligible."