The two-page explanation, which was published in seven languages and approved by Pope Francis, goes on to describe homosexuality as a “choice”.
It reads: “In this case, in fact, the blessing would manifest not the intention to entrust such individual persons to the protection and help of God, in the sense mentioned above, but to approve and encourage a choice and a way of life that cannot be recognised as objectively ordered to the revealed plans of God.”
The decree said gay people must be treated with dignity and respect, but that gay sex is “intrinsically disordered.”
Catholic teaching says that marriage is a lifelong union between a man and woman, is part of God’s plan and is intended for the sake of creating new life.
The Vatican statement has led to criticism of Pope Francis, who has publicly looked to adopt a more inclusive tone towards LGBT people since he was elected in 2013.
New Ways Ministry, an organisation based in the US which supports gay people within the Catholic church, said the decision was “not surprising, but still disappointing”.
It tweeted: “The Vatican’s decision on blessings for same-gender unions is an impotent one because it won’t stop the movement to bless such couples.
“It is not surprising, but still disappointing, that the Vatican has responded “no” to the dubium about whether the church can bless same-sex unions.
“Though Rome has now spoken on this issue, what the Vatican doesn’t realize is that the Catholic faithful are not satisfied with the answer that they gave.”