Gay groups and activists have reacted angrily after Pope Benedict XVI said that mankind needed to be saved from a destructive blurring of gender.
Speaking on Monday, the Pope warned that blurring distinctions between male and female could lead to the "self-destruction" of the human race.
The comments were "irresponsible and unacceptable", the UK's Lesbian and Gay Christian Movement (LGCM) said.
Vladimir Luxuria, a transgender former Italian MP, called his words "hurtful".
The row erupted as news emerged that the pontiff is to pay his first visit to the Holy Land in May next year.
Pope Benedict made the comments in an end-of-year speech to senior Vatican staff.
Defending God's creation was not limited to saving the environment, he said, but also about protecting man from himself.
It was not "out-of-date metaphysics" to "speak of human nature as 'man' or woman'", he said. It came from the "language of creation, despising which would mean self-destruction for humans".
Gender theories, he said, led to man's "auto-emancipation" from creation and Creator.
"Rain forests deserve, yes, our protection but the human being... does not deserve it less," he said.
LGCM head Rev Sharon Ferguson said the Pope's remarks justified "gay bashing" and bullying.
Mark Dowd, strategist for Christian environmental group Operation Noah, said the comments betrayed "a lack of openness to the complexity of creation".
And Ms Luxuria, who recently lost her seat in the Italian parliament, said suggesting people like her were destructive was very hurtful.
"I'm someone who was born as male and has a spiritual and female soul, and it's contradictory that a Pope just thinks of people just made as flesh and not made of a spiritual aspect."
The Catholic Church opposes gay marriage. It teaches that while homosexuality is not sinful, homosexual acts are.
Earlier this month, the Vatican said that a proposed United Nations resolution decriminalising homosexuality went too far.
"Unjust discrimination" against gay people should be avoided, but the use of wording such as "sexual orientation" and "gender identity" in the text would "create serious uncertainty in the law", it said.
This article has been amended to make it clear the Pope made no direct reference to homosexuals or transsexuals.