Crawley MP Henry Smith will vote against plans to legalise gay marriage after more than 300 constituents wrote to him with concerns.
He said changing the definition of traditional marriage would cause a great deal of upset and stress to a large number of people.
Mr Smith said: “Gay relationships are a reality. I know a lot of gay people and I have no problem with that lifestyle. But not everyone shares that view. Some people profoundly disagree with homosexual relationships.
“Redefining a word in language will cause a great deal of upset and stress to a large number of people.”
The Government will put proposals to make marriage between heterosexual and homosexual couples equal before parliament in the new year.
MPs will be free to vote as they please with more than 100 Tories saying they are against the change.
But with wide support among Labour and the Lib Dems the government is set to push the change through.
Mr Smith said only two or three Crawley constituents had written to him in support of a change to the law while more than 300 had expressed deep concerns over the prospect.
He argued that civil partnerships providedthe same legal rights as marriage and that a change in the law would inevitably result in churches and mosques being forced to conduct gay marriages against their religious beliefs.
He added: “I think it’s wrong that faith groups should be forced to accept same sex marriages. When it comes to matters of faith it should be a matter of conscience.”
“If parliament redefines the word marriage it unnecessarily offends people of faith that hold a great deal of store in the concept of marriage.”
Prime Minister David Cameron and Deputy Prime Minister Nick Clegg have said they support allowing religious institutions to decide for themselves whether they want to conduct same-sex marriages.
The priority given to the vote is thought to be part of a bid to make the Conservative Party more gay-friendly following Section 28, a 1980s law which prevented the promotion of homosexuality in schools.
Cllr Chris Oxlade (Lab, Ifield) said the Labour Party had spearheaded the drive towards equality by introducing civil partnerships and repealing section 28 which had vilified and victimised gay people.
He said: “I’m genuinely surprised Henry isn’t backing the idea as he has always been an advocate for gay rights. This is a genuine drive towards equality and the Labour Party is supporting this change in the law”
Cllr Liam Marshall-Ascough (Con, Southgate) disagreed with Conservative colleage Henry Smith.
He said: “It’s wrong to make a distinction between groups in society.
“I lived in apartheid South Africa where you couldn’t marry someone from a different race. Over here we’re saying you cannot marry someone because of who you are.
“The only people it matters to are those getting married, it has no relevance to anyone else.
“The state shouldn’t interfere with how people live their lives. You don’t choose who you love or who you are.”