The Catholic Church's leader, Pope Benedict XVI has called gay marriage laws an "attack" on the natural differences between men and women.
Speaking just after Portugal's parliament voted to legalise same-sex marriage, the pontiff addressed the comments to the Vatican diplomatic corps in a message which focused on environmental issues.
This is not the first time he has used environmental messages to preach about the supposed sins of gay people.
In an end-of year address in 2008, he said that the existence of gay people threatens humanity as much as the destruction of the rainforests does and that "blurring" genders through acceptance of transgender people would kill off the human race.
According to AFP, he spoke about protecting or endangering creatures including humans in today's address and said: "One such attack comes from laws or proposals which, in the name of fighting discrimination, strike at the biological basis of the difference between the sexes."
He cited "certain countries in Europe or North and South America", which is assumed to mean Portugal and Mexico City, which legalised gay marriage last month.
The Pope is due to visit the Catholic country of Portugal in April, a month after a law allowing gay marriage is expected to come into force.
He is also expected to visit the UK this year, and humanist groups have already announced their plans to protest.
Similar demonstrations were held when Pope John Paul II visited Britain in 1982.
In March, the Pope provoked anger when he spoke out about his view on HIV.
While on a flight to Africa in March, he told journalists that condoms "aggravate" the problem of HIV. He was roundly condemned for the statement, while respected medical journal The Lancet demanded that he retract the comments.
The Pope has previously counselled that abstinence is the only way to counter the spread of the disease.