A Conservative MP has suggested fewer heterosexual couples will get married if gay marriage is introduced.

Speaking in the Commons on Tuesday after culture secretary Maria Miller unveiled the government plans for same-sex marriage, Rugby MP Mark Pawsey said the move would lead to more unmarried parents.

"The secretary of state herself referred to the institution of marriage as a building block of our society and yet last year nearly half of all babies were born to couples who were not married," he said.

To the bafflement of many MPs he added: "Given that marriage rates in Spain and Holland collapsed after same sex marriage was introduced there, is she not concerned that even fewer couples intending to have children will choose to get married?"

Fellow Tory Penny Mordant, replied: "As an umarried heterosexual woman, if the Bill goes ahead, I will certainly consider the institution."

The questions to Miller exposed the depth of opposition on the Tory benches to gay marriage.

Matthew Offord suggested it opened the door to polygamy and Laurence Robertson said people were "deeply offended" by the idea of gay people getting married.