The Christian Legal Centre has criticised David Cameron over plans to introduce an Equal Civil Marriage Bill to Parliament next week.
The Prime Minister said the Bill would allow churches to host same-sex weddings on their premises.
Mr Cameron said today that he was a "massive supporter" of marriage and did not want gay people "to be excluded from a great institution".
He also insisted that churches would not be forced to conduct gay marriages if they did not want to.
"But let me be absolutely 100% clear, if there is any church or any synagogue or any mosque that doesn't want to have a gay marriage it will not, absolutely must not, be forced to hold it," he said.
Mr Cameron added that MPs would have a free vote on the issue.
His assurances of church protection, however, have failed to convince the CLC, which provides legal support to Christians experiencing discrimination.
CLC director Andrea Minichiello Williams said: "If this moves ahead the courts' interpretation of equality legislation will not provide any effective protection from litigation for churches who do not wish to perform such ceremonies, whatever the Prime Minister says now. Any such assurances are meaningless.
"At the Christian Legal Centre we have seen countless cases where Christians have been forced out of their jobs for their refusal to condone and promote homosexual practice. Their views have not been respected or accommodated and Mr Cameron has ignored their plight.
"This does not bode well for British Christians if further legislation is passed. Assurances to churches who do not wish to perform same-sex 'marriages' fly in the face of all the evidence."
The CLC has itself faced difficulty because of its defence of traditional marriage.
A marriage conference organised by the organisation earlier this year almost had to be cancelled when two venues - the Law Society and the Queen Elizabeth II conference centre - pulled out of hosting it at the last minute.
Both centres said the bookings had been cancelled because the CLC's views on marriage contravened their equality policies.