Peter Tatchell had previously supported the legal action again Ashers Bakery in Northern Ireland, and celebrated the verdict that it had discriminated against a customer.
It refused to bake a cake with the Sesame Street characters Bert and Ernie with the words 'support gay marriage' above them.
Mr Tatchell now says he's changed his mind and has called for Ashers to be acquitted of discrimination when its appeal hearing starts on Wednesday.
He has has said while there should be no discrimination against people themselves, people should be allowed to refuse to promote a message they disagree with.
The campaigner also said that if the discrimination decision against Ashers Bakery was upheld, it would have significant implications for other businesses.
For example, a Muslim printer may have to publish cartoons of the Prophet Muhammad, or a bakery owned by a gay couple may have to make a cake with anti-gay slogans on it.
Mr Tatchell told Premier's News Hour: "In a free and democratic society people have a right to not promote political messages that they conscientiously object to.
"In this particular case Ashers did not refuse to serve Gareth Lee because he was gay. It was because of the message on the cake - and that's a very important distinction.
"Some people have said that I'm mistaken, that I've betrayed the cause of equality. Whether I'm right or wrong, I'm following my conscience.
"I did take a particular stand against Ashers which I now recognise to be wrong."
Listen to Premier's Antony Bushfield speaking to Peter Tatchell on the News Hour:
