Sex could be removed from the law of marriage if same-sex marriage gets the go ahead.
Under current rules, a marriage may be annulled if a man and woman do not consummate the marriage with sexual intercourse.
The details of the law mean that same-sex couples cannot meet the requirement, and the law may be altered if marriage is redefined.
Tory MP Edward Leigh says such a move would create legal chaos for people caught in sexless marriages.
Currently non-consummation of a marriage is a ground for the union to be annulled and Mr Leigh warned such a change to the law will have “profound effects” in this area.
Last week it was revealed that Conservative MPs – including ministers – will be free to vote according to their consciences on the issue of redefining marriage.
It comes after Owen Paterson, the Northern Ireland Secretary, became the first Cabinet Minister to publicly oppose the Government’s plans to redefine marriage.
However on Sunday Nick Clegg made clear that his MPs will be forced into voting for the changes.
Mr Clegg said it was not a “matter of conscience”.
Last month The Sunday Times newspaper accused David Cameron of pushing for an “unnecessary” change over the issue.
And Ann Widdecombe has said she did not campaign for Mr Cameron “all day, every day in the last general election” so that he could destroy traditional marriage.
Earlier this month a homosexual writer warned that activists seeking to rewrite the definition of marriage must stop trying to vilify those who oppose them.
“If they don’t snap out of it they’ll even lose the support of gay people like me,” said Max Wind-Cowie.
He is Head of the Progressive Conservatism Project at Demos and he accused the “gay-rights brigade” of allowing their quest to redefine marriage to turn “into active hetero-phobia”.