The Marriage (Same-Sex Couples) Bill is set to "isolate" young Christians who believe in the traditional definition of marriage, church leaders have warned.
In a letter to The Telegraph, they said young people would think twice about taking public sector jobs after gay marriage has been legalised.
The bill is having its third and final reading in the House of Commons. The Government struck a deal with Labour on Monday to defeat an amendment that would have delayed gay marriage coming into law.
Today MPs will debate further amendments and make a final vote on Tuesday night. Once passed, the bill will go to the House of Lords on Wednesday, where it faces tough opposition.
Church leaders said young people from teeangers to those in their thirties "will suffer discrimination and face new risks to their careers and futures".
Signatories of the letter include the Archbishop of Southwark, the Right Reverend Peter Smith, Rector of St Ebbe's, Oxford, the Reverend Vaughan Roberts, Rector of All Souls, Langham Place, the Reverend Hugh Palmer, and the National Director of the Fellowship of Independent Evangelical Churches, the Reverend John Stevens.
Together the church leaders' congregations number 150,000 people, some 50,000 of whom they said were aged between 13 and 30.
"For many in this rising generation, marriage is the union of sexual opposites, and the thread that binds generations," the letter continued.
"If the Bill passes into law without much clearer protections for freedom of speech and freedom of belief, teachers and public-sector workers will have to choose between their conscience and their career, as many will be deterred from a public-service career or from charity involvement.
"The Bill is supposed to be pro-marriage, pro-equality and pro-diversity, yet, as drafted, it is none of those things. There will be anger and sadness, and this Bill will cause pain for many, without tackling prejudice against the few. We, and many young people in our congregations, are concerned about the consequences if it is passed in its current state."