The image of the rosy-cheeked newborn, with the slogan "Sexual orientation is not a choice", will soon go up on billboards across Tuscany as part of a drive by its regional government to curb discrimination against gays and lesbians.
The Vatican and conservative politicians were quick to criticise the advertisement, images of which have already appeared in Italian newspapers.
Christian Democrat MP Luca Volonte said using a newborn to suggest that homosexual tendencies were innate was "misleading and shameful".
Opposition Senator Maria Burani Procaccini said heads should roll over the matter.
The Vatican, which does not consider homosexual tendencies sinful but condemns homosexual acts, called the matter strange.
"There is no need for an advertisement of this kind," the Vatican's top diplomat and Secretary of State Cardinal Tarcisio Bertone told reporters.
Italy's main gay rights group Arcigay said the ad was at the vanguard of the struggle for equal rights.
"I'm very much in favour of the advertisement because it expresses a concept that I've been convinced of for some time - that homosexuality is not a choice," said gay MP Franco Grillini.
"The only choice possible for a homosexual is to accept their sexuality or live unhappily."
Not all gay activists were enthralled. Philosopher Gianni Vattimo said the ad risked suggesting gays and lesbians were a race apart.
Transgender leftist MP Vladimir Luxuria said he was perplexed by the choice of the image.
The brainchild of Canadian foundation Emergence, the controversial campaign was used previously in Quebec.
Consultant Agostino Fragai was involved in picking the campaign for the wealthy and historically progressive Tuscany region, and he said he knew the image was provocative, but felt it was important to raise awareness.
"We chose it because it was a strong but tender poster, with a baby in it," Fragai said.
The region was not interested in joining the debate on whether sexual orientation was determined at birth, he said.
"This is a poster, not some scientific treatise."