The Pope explains his reasoning and it is quite simple.
Via The Telegraph:
"In the fight for the family, the very notion of being – of what being human really means – is being called into question," the Pope said in Italian during an end-of-year speech.
"The question of the family ... is the question of what it means to be a man, and what it is necessary to do to be true men," he said.
The Pope spoke of the "falseness" of gender theories and cited at length France's chief Rabbi Gilles Bernheim, who has spoken out against gay marriage.
"Bernheim has shown in a very detailed and profoundly moving study that the attack we are currently experiencing on the true structure of the family, made up of father, mother, and child, goes much deeper," he said.
He cited feminist gender theorist Simone de Beauvoir's view to the effect that one is not born a woman, but one becomes so – that sex was no longer an element of nature but a social role people chose for themselves.
"The profound falsehood of this theory and of the anthropological revolution contained within it is obvious," he said.
The defence of the family, the Pope said, "is about man himself. And it becomes clear that when God is denied, human dignity also disappears."