Pope Francis has opened a divisive meeting of the world's bishops on family issues by asserting that marriage is a binding bond between a man and woman.

He said, though, the church does not judge and must "seek out and care for hurting couples with the balm of acceptance and mercy".

Francis dove head-on into the most pressing issue confronting the meeting of 270 bishops during a solemn Mass in St. Peter's Basilica.

He insisted the church cannot be "swayed by passing fads or popular opinion".

But in an admission that marriages fail, he said the church is also a mother who does not point fingers or judge her children.

One of the major debates at the synod is whether divorced and civilly remarried Catholics can receive Communion.

Francis launched the synod process two years ago by sending out a 39-point questionnaire to bishops, parishes and ordinary Catholic families around the world asking about their understanding of and adherence to church teaching on family matters.

Their responses showed a widespread rift between official Catholic teaching and practice, particularly on sex, marriage and homosexuality.

A mid-level official in the Vatican's orthodoxy office, Monsignor Krzysztof Charamsa, announced on Sunday that he was a gay and had a boyfriend. He called for the synod to take up the plight of gays and denounced homophobia throughout the church.

The Vatican summarily fired him.

Former Irish president Mary McAleese, a practising Catholic with a gay son, said she hoped that more transparency would help "kill for once and all this terrible lie" that everyone was born heterosexual.