The Christian Institute has come out in opposition to the Government's plans to hold a consultation on whether to change the legal definition of marriage.
Equality Minister Lynne Featherstone announced last weekend that the Government would launch the consultation next spring.
At present, only unions between a man and a woman are defined as a 'marriage'.
Gay couples are granted similar legal protection to married couples when they enter into a civil partnership, but are not allowed to marry.
Similarly, a heterosexual man and woman may not enter into a civil partnership with one another.
The Government suggested a change to the law could be in place by 2015, although it made clear there were no plans to change the rules on civil partnerships.
Mike Judge, of the Christian Institute, questioned the need to redefine marriage to include unions between homosexuals.
"All the legal rights of marriage are already available to homosexual couples through civil partnership registrations," he said.
"The census shows that only one household in 500 is headed by a same-sex couple. Is marriage to be redefined across the whole of society to please a small minority of activists?"
The Prime Minister has reportedly personally intervened to speed up progress on allowing homosexuals to marry.