The Vatican's highest-ranking expert on Church law, Cardinal Francesco Coccopalmerio, was speaking at the family synod in Rome.
He was asked if there could ever be a time when the Church would accept gay marriage and even offer blessings.
"We have to be honest," he said.
"For us, and not just for us but for human culture in general, marriage is between a man and a woman."
He added that the Church would not judge people in gay relationships, but said: "To bless this type of union, to say that they are like [heterosexual] marriages, never.
"This is simply for reasons of logic and identity. To bless them is not part of the way we see Christian doctrine."
There have been calls ahead of the synod for the Church to change some of its rules regarding gay relationships, but Cardinal Coccopalmerio rejected that.
He said: "There can be exquisite people in these conditions [being gay], but that's not the same as saying that this is a good union and should be blessed. That is something else."