One unnamed local Tory association has seen membership nose-dive from 850 to 550 largely because of gay marriage, the Daily Express reports.
It follows news that big money donors have stopped giving to the party because of the Prime Minister’s obsession with the issue.
There have been numerous resignations of senior local activists, and widespread grassroots anger at David Cameron’s plans.
In the wake of the Eastleigh by-election defeat, Tory MP Stewart Jackson said disillusioned party activists had stayed at home.
He said: “This is the price that has to be paid for gay marriage because of the drastic unpopularity of it with activists and supporters, who have been less inclined to get out and campaign on the streets because of it.
“The issue has been a direct recruiting seargeant for UKIP, and there’s anecdotal evidence across the country for UKIP.”
Last month, a Tory chairman in David Cameron’s own constituency resigned because of the Prime Minister’s support for redefining marriage.
Cicely Maunder said she had “no idea” why Mr Cameron supports the change, and said other local members also walked out over the plans.
A few days earlier, a Conservative councillor in Henley left the Party, saying it has no mandate for meddling with marriage.
In January, a Tory constituency chairman quit the party after 49 years over gay marriage, calling it an “ill-thought-out” idea.
Edmund Costelloe had been the chairman of Somerton and Frome Constituency Conservative Association.
He said: “David Cameron and a group of colleagues have utterly lost touch with the grassroots of the Conservative Party, the majority of whom are outraged by this attack on natural and historic family values.”