Bashir Maan, the former Glasgow Labour councillor and spokesman for the Council of Glasgow Imams, also said it would lead to a decline in world population as governments "promote homosexuality".
His comments come as Glasgow imams urge Muslim voters to boycott May's local elections if they cannot find a candidate who opposes gay marriage.
It would be better to withhold a vote rather than cast it for a candidate in favour of same-sex marriage, they argue, despite all of the main parties supporting the idea.
A key fear among imams is that mosques could be sued or prosecuted if they refuse to conduct gay marriages, despite the Government insisting no-one will be compelled to conduct ceremonies.
One worshipper, attending a mosque in Glasgow's west end on Friday, confirmed the imam had been going "hell for leather" on the subject.
The SNP Government is currently analysing about 50,000 responses to its consultation on whether to allow same-sex couples to have full religious marriages, rather than just civil partnerships.
Ministers are "minded" to make the change, and Alex Salmond says he is personally in favour provided no church is forced to marry gay couples.
Maan said Glasgow imams would be delivering their message in sermons ahead of the May 3 vote. About 40% of Scotland's Muslims live in Glasgow.
"Religious people only advise," Mann said, "but as this [homosexuality] is considered as a grave sin in our religion we have got to do everything possible to try to oppose it."
He said the political consensus on gay marriage was denying the electorate a proper choice. "They're all in it. You cannot pinpoint one party. It's political correctness gone mad. What purpose would it serve that is not already being served by the civil union?
"The procreation process will stop if people started marrying same-sex people. Would the procreation process carry on? No, it would stop. Not everyone, but it would make a difference."
He added: "If this trend goes on increasing, of homosexual unions, what will become of society? Where would civilisation end up? A decline in population, and a decline in society.
"Society is based on the family. The family of a man and a wife and the children, that is the basis of family. If that's taken away, it will crumble eventually. Society will crumble."
When it was put him to that society has proven fairly resilient until now, he said: "So far, governments have not been promoting homosexuality so much. Now it's different. If the Government thinks it's a good thing, then maybe their expectations are different.
"It is a sin in not only our faith but in Christianity and other faiths also. No faith approves or condones homosexuality."
Tom French, of the gay rights group Equality Network, said the Glasgow imams were trying to "threaten" the Government ahead of the local elections. He said: "It's not very helpful. The clear intention is to make the Government feel as though they will be less likely to win the local elections if they go ahead with this. But councillors won't be making the decision on this, so it seems more like a punishment than a reasonable step."
One veteran SNP campaigner in Glasgow, who asked not to be named, said the party was avoiding the issue on the campaign, adding: "It's one of those 'Don't talk about the war' things."
Hanif Raja, Labour's candidate in the Glasgow ward of Pollokshields, said gay marriage was an issue for national rather than council elections.
"I haven't come across it as an issue and don't think it will make a difference in how people vote," he said.
Shabbar Jaffri, the SNP candidate in nearby Greater Pollok, who carried out extensive work on the Government consultation, said the consensus in the Muslim community was that legalising gay marriage was not needed – a view shared by the Jewish community, Catholic Church and the Kirk. However, he said it was not an election issue.
"I have been campaigning in the same community and I can honestly say not one person has brought it up. The issues that are being discussed are things like the business bonus scheme, rates relief, children at university and tuition fees.
"We're living in a cosmopolitan society. The Asian community has first-hand experience of prejudice. I don't view the gay community in a negative way or think they should be chastised or done away with. It's a fact of reality whether you like it or not, and you have to accommodate people."
Glasgow SNP MSP Humza Yousaf added: "Every imam I have spoken to thus far understands that religious freedom cuts both ways. While legal safeguards for Mosques and those who do not want to solemnise same sex marriage are vital, there is an understanding from imams that what others wish to do in their faith institutions is entirely their own business."