The document reaffirms Catholic teaching on marriage and sexuality when several liberal bishops, including the head of the German Catholic bishops’ conference, have called for blessing same-sex couples in committed relationships. Priests in Germany have widely blessed such couples for years, as have clergy in some other parts of Northern Europe.

Such blessings are wrong, the Vatican said on Monday, because they would seem “to approve and encourage a choice and a way of life that cannot be recognized as objectively ordered to the revealed plans of God,” adding that God “does not and cannot bless sin.”

German bishops have tangled with the Vatican on other matters, including the question of giving Communion to Lutherans, and are unlikely to back down in their stance on blessing gay unions. German bishops and lay Catholics are currently involved in a national synod that is considering changes to aspects of church life, including the possibility of women clergy and teaching on sexuality.

A move by German bishops to approve blessings of same-sex unions would exacerbate tensions with more conservative parts of the church, including in Africa and the U.S. Conservative bishops in the U.S. have been critical of what they see as an excessively progressive drift away from traditional teachings, with the archbishop of Denver warning in 2019 that the German bishops are moving toward a schism.

Pope Francis has taken a more liberal approach than his predecessors to some questions of marriage and sexuality, including divorce and homosexuality. In one of the most famous statements of his pontificate, he responded to a question about gay clergy in 2013: “Who am I to judge?” During his 2015 visit to the U.S., he met privately with a gay couple in Washington, D.C.

In comments published last year, the pope expressed support for same-sex civil unions, saying that gay couples “have the right to be legally covered,” a stance he had held as archbishop of Buenos Aires.

But the pope has also written that “there are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God’s plan for marriage and family.”

Monday’s Vatican document acknowledged “the presence in such relationships of positive elements, which are in themselves to be valued and appreciated,” but said such elements “cannot justify these relationships and render them legitimate objects of an ecclesial blessing, since the positive elements exist within the context of a union not ordered to the Creator’s plan.”

The Catechism of the Catholic Church, an official handbook of teaching, states that “homosexual acts are intrinsically disordered,” the inclination to perform them is “objectively disordered” and “under no circumstances can they be approved.” But the catechism also states that gay people “must be accepted with respect, compassion and sensitivity. Every sign of unjust discrimination in their regard should be avoided.”

Monday’s reaffirmation of traditional teaching is likely to disappoint progressive Catholics hoping for further change and cheer conservatives, as did the pope’s decision last February not to make it easier to ordain married men to the priesthood.

“It is not surprising by still disappointing,” said Francis DeBernardo, executive director of New Ways Ministry, which advocates for LGBT Catholics. “This decision though is an impotent one because it will not stop the Catholic people in the pews, nor many Catholic leaders, who are eager for such blessings to happen.”

The question of homosexuality has roiled other Christian denominations, fomenting division with the world-wide Anglican Communion between liberal churches in Europe and North America and more conservative churches in Africa. Last year, the United Methodist Church agreed in principle to split because of disagreements over same-sex marriage and gay clergy, though a meeting to approve the move has been delayed because of the pandemic.

Write to Francis X. Rocca at francis.rocca@wsj.com