The concept of adultery could be removed from the law if the Government pushes ahead with its plans to redefine marriage, MPs and lawyers have warned.
In the controversial Marriage (Same Sex Couples) Bill, only unfaithfulness between people of opposite genders counts as adultery.
Ayesha Vardag, a leading London divorce lawyer, said that the difference between heterosexual and homosexual marriage law could now pave the way for adultery to be abolished.
She said: “The only appropriate and balanced way to deal with this would be to scrap the act of adultery as a basis for divorce altogether,”
She added: “This might indeed be the point at which adultery dies the death in order to rectify this imbalance.”
David Burrowes MP thinks the Government has “opened up a Pandora’s box”.
He said: “It has been a fundamental principle of marriage that it is based around exclusive sexual fidelity and that is what the redefinition of marriage would remove.”
He added: “This will create an opportunity, for good or ill, to recodify marriage which has developed in case law and has been in a settled position until now.”
Gay rights campaigners have also roundly criticised the gay marriage Bill for its unequal treatment.
Iain Dale, radio presenter and Conservative party commentator, said the fact same-sex couples will be unable to use adultery as a reason for divorce as “madness” and labelled the Bill “flawed”.
And homosexual activist Peter Tatchell says the proposals are unfair since heterosexual people are unable to have a civil partnership.
The first vote on the Marriage (Same Sex Couples) Bill takes place in the House of Commons on 5 February.