The posters, by the Core Issues Trust, stated: "Not gay! Post-gay, ex-gay and proud. Get over it!"
The adverts were backed by Anglican Mainstream, a global group that says it is committed to the traditional biblical teaching on marriage, the family and human sexuality.
They were believed to mock pro-gay group Stonewall's adverts that said: "Some people are gay. Get over it."
Anglican Mainstream has now been in touch with solicitors about the legality of the ban, because it had already been cleared before Mr Johnson became involved.
Tom Ellis, from law firm Aughton Ainsworth, said he had been contacted by the group with a view to taking legal action against Transport for London (TfL) because the advert had already been cleared by the Committee of Advertising Practice.
The solicitor is now seeking an explanation from TfL with their justification for the ban. He said: "It seems there is a decision that has been made that can be challenged."
The posters were due to run for two weeks on the sides of vehicles serving main routes through the capital, including close to St Paul's Cathedral, Oxford Street, Trafalgar Square and Piccadilly Circus.
But London Mayor Boris Johnson ordered it to be pulled after the campaign was labelled as homophobic.
Mr Johnson, who is standing for re-election next month, said: "London is one of the most tolerant cities in the world and intolerant of intolerance.
"It is clearly offensive to suggest being gay is an illness someone recovers from and I am not prepared to have that suggestion driven around London on our buses."
The doomed campaign, which was also backed and cleared by industry regulator the Advertising Standards Authority, claimed therapy could change sexual orientation.
Core Issues' co-director Mike Davidson criticised the decision to axe the adverts, saying: "I didn't realise censorship was in place.
"We went through the correct channels and we were encouraged by the bus company to go through their procedures. They okayed it and now it has been pulled. I would be interested to know on what basis they have done that."
He added: "It is of deep concern that there can only be one point of view and that is the point of view of individuals who are determined to push through gay marriage and apparently believe that homosexuality cannot be altered in any possible way.
"That is not a universally held view. This is a disturbing development and it is disappointing the UK finds itself in this position."
But Stonewall spokesman Andy Wasley told Sky News he welcomed the move: "We are delighted by TfL's (Transport for London) clear commitment to diversity. It is fantastic that no adverts will be promoting voodoo, gay-cure therapy in London."
:: Sky News is hosting the first televised debate of the London Mayor campaign, with Mr Johnson, Labour candidate Ken Livingstone and Liberal Democrat candidate Brian Paddick, on April 19.
London Mayoral Candidates
:: Siobhan Benita (Independent)
:: Carlos Cortiglia (British National Party)
:: Boris Johnson (Conservative)
:: Jenny Jones (Green)
:: Ken Livingstone (Labour)
:: Brian Paddick (Liberal Democrats)
:: Lawrence Webb (UKIP)