Amid the backbencher opposition to same-sex marriage, Tory MP Mark Pawsey (Rugby) said the proposals could impact whether parents or people planning to have children choose to marry.
He said nearly half of the babies born in the UK had parents who were not married and added:
Given that marriage rates in Spain and Holland collapsed after same-sex marriage was introduced there, are you not concerned that even fewer people intending to have children will choose to get married?
Same-sex marriage will remain illegal in the Church of England, as the Government unveiled its plans to extend marriage to gay couples.
David Cameron is facing a backlash from Tory MPs and religious groups over allowing gay weddings in churches.