Benedict's remarks came during his greeting to the Roman Curia (the papal court), Italy's ANSA news agency reported.
The pope said God's natural design of family was founded on the marriage of a man and a woman, without whom "figures fundamental to human existence disappear: the father, the mother, the child," and without whom "essential dimensions of the experience of being a person fall."
Benedict's comments came just about a week after he said publicly he believes gay marriage could cause "serious harm to justice and peace."
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