UK gay rights group Stonewall is calling for equality campaigners to back the gay marriage consultation.
Last week, the Home Office launched a 12-week public consultation on gay marriage.
The proposals would allow partners to have a civil wedding and take the same vows and commitments as opposite sex couples.
However, the plans have faced fierce opposition from both religious leaders and a minority of members of the Conservative Party.
Stonewall says it is pleased that the consultation process has finally started and is urging all supporters of equal marriage to make their voices heard by responding to the consultation before it closes on 14 June.
Chief Executive Ben Summerskill said: ‘Leading clerics and some political figures have, in the past three weeks alone, compared equality for gay people with slavery, child abuse, polygamy and bestiality.
'That’s why it’s critical that the calm voices of lesbian and gay people are heard in this debate too.'
Stonewall last month published a draft parliamentary bill for giving married same-sex couples equal legal rights, outlining the legislative steps needed to implement a policy now supported by all political party leaders and saying a gay marriage bill is so simple, it could be included in the Queen's Speech.
‘Ministers have pledged that same-sex couples will be able to marry in Britain by 2015,' Summerskill added.
'We trust that, given this commitment, these proposals will be included in the Queen’s Speech in May.’