The Grassroots Conservatives group says the same-sex marriage bill has seen many Tory supporters leave the party.
It wants the Prime Minister to introduce a bill to support UK Christians and help "kick-start the healing process".
Robert Woollard, chairman of the group, said: "Mr Cameron must kick-start the healing process with ordinary members and voters who have felt marginalised and ignored if he is to stand any chance of winning the next election.
"He has for too long allowed the policy agenda to be driven by the so-called rebranding exercise, which ditched tried and tested policies, like cutting taxes and backing married families in favour of covering the country in wind farms and introducing gay marriage.
"This has not as his advisers predicted won lots of extra votes; in fact, it has had the opposite effect by splitting the right and pushing hundreds of thousands to the purple peril, while bleeding associations dry of members and activists."
The group, which isn't affiliated to the Conservative Party, says that following the Clacton by-election result (which UKIP defector Douglas Carswell won), the damage done to the Party by the same-sex marriage policy can no longer be ignored.
In a letter to David Cameron, Robert Woollard said: "To secure the necessary lead you need to win back those you have alienated. In the May 2013 ComRes poll, 28% voters cited SSM as the reason they would not vote Conservative in the upcoming elections. Furthermore, 27% of those aged 18-24, and 35% aged 25-34 stated that SSM would NOT help us in the elections."
"If you really do believe, as you profess, in the importance of the Christian heritage of the country, the centrality of the family, religious freedom and freedom of conscience in a democracy then you should respect the fact that a sizeable percentage of the population including Christians and many faith based ethnic groups are looking for such conscience clauses. Failure to provide for them in the manifesto will alienate them further and seriously risk losing the election for you, and more importantly will hand Ed Miliband the keys to Downing Street."
Some argued the new legislation on same-sex marriage would cause problems in the workplace for those who do not support it.
Concerns were raised for teachers, chaplains and registrars, although no cases have been brought to light so far.
Premier has contacted the Government Equalities Office and is waiting for a response.