A vote in the House of Commons to extend same-sex marriage plans leaves no guarantees for freedom of speech or worship, the Coalition for Marriage has warned.
It fears that church ministers may be penalised if they refuse to conduct gay marriage services, and that public sector workers with conservative views could lose their jobs.
MPs voted overwhelmingly to introduce same-sex marriage to Northern Ireland if its devolved Parliament is not restored by October 21.
Stormont has been suspended since January 2017 but the political stalemate is not expected to end soon.
The Coalition for Marriage decried the amendments passed in the Commons last week, saying that a "coach and horses were driven through devolution".
It criticised the plans to introduce gay marriage to the province through the Northern Ireland (Executive Formation) Bill and contended that such changes to the law should instead be given their own Bill and debate time in Parliament.
"Why is it that supporters of same-sex marriage so often resort to underhand tactics?" the coalition said.
"If redefining marriage is so great, why can't the case be argued?"
It questioned whether those who disagree with gay marriage in Northern Ireland will be able to voice criticism in public without facing repercussions.
"Because of the breakneck speed there are no specific guarantees for freedom of speech and freedom of worship in the Province," it said.
"We will do all we can to protect the freedom to criticise same-sex marriage in Northern Ireland."
It warned that the Equality Commission's case against the Ashers Baking Company for refusing to bake a pro-gay cake did not bode well for opponents of same-sex marriage.
"Northern Ireland's Equality Commission won't help. They spent four years pursuing Ashers Baking Company through the courts just because they opposed same-sex marriage," the coalition warned.
"All this goes to show the Province needs protection to speak against same-sex marriage just as there is in the rest of the UK. They don't have it in law at the moment.
"Teachers and others in the public sector must not be damaged in their careers just because they don't believe in same-sex marriage. And ministers of religion must be free to refuse to conduct homosexual marriage services."