The Prime Minister's described the Tories' performance at the Eastleigh by-election as disappointing.
The Liberal Democrats held on to the seat - albeit with a reduced majority - while UKIP beat the Conservatives into second. David Cameron blames tactical voting. Kevin Milburn ran for the Christian Party and told Premier's News Hour he thinks gay marriage was a key factor when it came to votes.
The Coalition for Marriage (C4M) has also said the Conservative Party's third-place finish is a reaction to the proposals to redefine marriage.
Mr Hart, Campaign Director for C4M, said:
"In 2010 none of the three main political parties had redefining marriage in their manifesto.
"UKIP's stunning second place result in this by-election should serve as a wake-up call to the Prime Minister."