Gay rights campaigner Peter Tatchell has accused Stonewall of attempting to “minimise” support for gay marriage equality.
Mr Tatchell claimed that a survey sent to the charity’s supporters was “flawed and unfair”.
Stonewall has come under pressure recently to speak out for marriage and civil partnerships to be opened to both gay and straight couples.
Earlier this month, current and recent Stonewall donors received an email from the charity asking them to rate its work over the last 12 months and what its priorities should be in the next five years.
Twelve options are given. One is “Civil partnerships – work to extend the legal form of marriage to gay people”. Other options relate to homophobic bullying, the treatment of gay people in the media and continuing to build the ‘Diversity Champions’ programme.
Stonewall argued last year for an amendment to the Equality Act to allow religious civil partnerships. This provision has not yet been implemented and remains under consideration by the coalition government.
Mr Tatchell accused the charity of not surveying its members adequately.
He said: “The question on gay marriage is buried near the end of the survey, as issue number eight. The question is not even called gay marriage. It is entitled ‘Civil Partnerships’, which is very confusing and misleading.
“This does not strike me as a genuine, impartial survey. It seems designed to minimise support for marriage equality.
“Stonewall is out of touch. The vast majority of the gay community and the British public support the right of same-sex couples to get married in a registry office.”
In response, a Stonewall spokesperson said: “Mr Tatchell does seem to be seriously underestimating the intelligence of gay people.”
Stonewall has come under pressure recently to campaign for gay couples to be given the right to marry in the UK.
Two of the group’s co-founders, Michael Cashman MEP and Ian McKellen, have called for it to lobby for the change.
Stonewall’s chief executive, Ben Summerskill, has said he will not be “jumped into” a position on changing the law before the charity’s 20,000 supporters are consulted.