Prime Minister Julia Gillard has pulled out of speaking at the Australian Christian Lobby's national conference, after its leader suggested a homosexual "lifestyle" was more hazardous to health than smoking.

The lobby's managing director, Jim Wallace, made the claim in Tasmania yesterday during a debate on same-sex marriage.

His comments outraged gay rights activists, who called for Ms Gillard to distance herself from the group by withdrawing from its conference, which will be held in Canberra on October 5 and 6.

Ms Gillard today branded Mr Wallace's comment ''offensive'' and said she would not speak at the lobby's conference.