Happily ever after doesn't require any complicated maths, the Vatican said on Tuesday -- for Catholics, one spouse is enough.

In a new decree approved by Pope Leo, the Vatican's top doctrinal office told the world's 1.4 billion Catholics they should seek to marry one spouse for life and should not have multiple sexual relationships.

Criticizing the practice of polygamy in Africa, including among members of the Church, the Vatican reiterated that it believes marriage is a lifelong commitment between one man and one woman.

The decree, which did not discuss same-sex relationships, focused on what it called the "richness and fruitfulness" of traditional marriage. It encouraged Catholics to find one spouse and become committed to them.

Marriage 'demands exclusivity'

"Every authentic marriage is a unity composed of two individuals, which requires such an intimate and totalising relationship that it cannot be shared with others," said the decree.

"Since (marriage) is a union between two people who have exactly the same dignity and the same rights, it demands exclusivity," it said.

The question of how to better enforce the Church's teachings on marriage was debated at two Vatican summits in 2023 and 2024, which the late Pope Francis hosted to discuss the future of Catholicism with hundreds of cardinals and bishops.

Polygamy in Africa, where many Catholics take part in long-standing cultural practices of maintaining more than one committed relationship, was a topic of heated discussion at those summits.

Also discussed was the rise of polyamorous relationship structures, where individuals date multiple people at the same time, in some Western countries.

"Polygamy, adultery, or polyamory are based on the illusion that the intensity of the relationship can be found in a succession of faces," the new decree said.

The document does not discuss divorce, which the Church does not recognise as it views marriage as a lifelong commitment.

The Church however has an annulment process, which evaluates whether marriages were properly contracted, and stresses that partners are not expected to stay in abusive relationships.