The two-week "Not gay! Post-gay, ex-gay and proud. Get over it!" campaign on London buses was due to start next week. It would have provided an alternative view to homosexual campaign group Stonewall's "Some people are gay. Get over it" adverts.
Mr Johnson's stated reason for banning the adverts is extraordinary for a democratic politician standing in a free election:
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.
It is truly frightening that Mr Johnson is not prepared to leave Londoners themselves to make up their own minds on the question of gay conversion. Does he believe that his electors are incapable of deciding on whether they think that homosexuality is an innate condition or that sexual orientation is more fluid and can therefore be subjected to the exercise of moral choices?
Moreover, cannot Londoners decide for themselves that if homosexuality is an innate orientation with which some people are born, individuals can choose on religious grounds not to act on their sexual desires and remain celibate?
Does Mr Johnson wish to impose on everybody the permissive society's view that sexual activity is essential to leading a fulfilled life?
Voting in an election - and Mr Johnson is standing in one next month for the office of London Mayor - involves the exercise of independent judgement. It involves weighing up the merits of various arguments. It involves exercising moral choices.
Reacting to Mr Johnson's decision, Core Issues' co-director Mike Davidson rightly identified the threat to democracy:
I didn't realise censorship was in place...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.
The threat to the proclamation of the Lordship of Christ in British society is manifest from Mr Johnson's decision in the UK capital. Christianity spreads by persuasion. It thrives in an atmosphere of rigorous and free philosophical and moral debate.
Indeed, that persuasive, argumentative aspect of the biblical Christianity that emerged following the 16th Century Reformation in England and Scotland contributed to the development of Parliamentary democracy on this island.
How tragically ironic that a British politician aspiring to office through a democratic election should collude with sinister forces that want to censor the proclamation of the Christian worldview on an issue of personal morality.