The definition of marriage could be re-written by Parliament if David Cameron succeeds in allowing homosexual couples the same rights as men and women.
Last year the Prime Minister said he didn't support gay marriage despite being a Conservative, he supported it because he was a Conservative.
Mr Cameron had been planning to allow gay couples to get married in civil ceremonies before the next election in 2015.
But mounting opposition from within his own party has forced him to bring the vote forward. It will be a free vote in the House of Commons, meaning MPs will not be tied by a party line and will instead vote according to their own individual views.
While some MPs, like Dudley North Labour MP Ian Austin agree with the Prime Minister, many among his own party are planning to defy him over allowing gay marriage.
Today they said they were concerned about the legal consequences for the church and about Parliament "re-writing" definitions that have been in place for hundreds of years.
Six West Midlands Conservatives are among 118 who have openly opposed the idea.
Conservative MPs against the idea of allowing gay marriage include Wolverhampton South West's Paul Uppal, South Staffordshire's Gavin Williamson, Aldridge and Brownhills MP Richard Shepherd, Stone's Bill Cash, Stafford's Jeremy Lefroy and The Wrekin's Mark Pritchard.
Gay couples can already have the same legal rights as married people with civil partnerships introduced in 2005.
Wolverhampton hairdressers Royston Blythe and Nick Malenko were among the first to get one.
The Civil Partnership Act means same-sex couples have the same property rights, exemptions on inheritance tax, social security and pension benefits as married couples.
One partner can also have parental responsibility for a partner's children.
But the Government wants them to be able to make marriage vows and have them recognised as being married in the same way as heterosexual couples.
Ministers have said that churches and other religious groups will be exempt but there are fears that they may end up forced to allow them because of human rights laws. So far 228,000 people have replied to the government's consultation on the idea – more than on any other issue since the coalition came to power in 2010.
In his speech to Tory party conference last year Mr Cameron said: "Conservatives believe in the ties that bind us; that society is stronger when we make vows to each other and support each other."
But two thirds of Tory voters believe marriage should continue to be defined as the union between a man and a woman, according to opinion polls.
Gavin Williamson, MP for South Staffordshire, said he was concerned about the idea of forcing the church into a legal battle against its deeply held beliefs. Even though the plans will relate to civil ceremonies he believes there will be a knock on effect for churches. He said: "If we were to legislate then the church could be forced against its will to allow gay marriages.
"I don't think it's right to put the church into the position where it faces a threat of legal action because it is adhering to its beliefs."
Stafford MP Jeremy Lefroy, a Christian, said he did not believe Parliament had the right to define the word "marriage".
He said: "I fully understand the importance to gay couples of having rights under civil partnerships.
"There are legal consequences that have to be looked at from redefining something that has been established for centuries. It's not for Parliament to redefine marriage."
Bill Cash said: "I am adamant in my opposition to gay marriage. It is a misuse of the word, procedure and ritual and I take the view that since time immemorial, marriage is the marriage of man and woman for the purposes of procreation – and that this is immutable."
Paul Uppal added: "My main concern is that inevitably this may lead to a situation where the church and other faith institutions will be placed in an invidious position.
"In my conversations with members of the lesbian and gay community, group marriage hasn't registered as an issue as they felt the issue of commitment was dealt with through civil partnerships.
"I believe that commitment, perseverance and community commitment are important values to stress at the moment and I'm not sure that a redefinition of marriage is actual and genuine progress."