Pope Francis Sunday defended marriage and heterosexual couples as he opened a synod on the family overshadowed by a challenge to Vatican orthodoxy by a gay priest.

Charamsa said his decision to come out was motivated by concern for the Church's attitude to homosexuals, which he described as "backwards".

The Vatican confirmed Friday that Francis had hugged an old gay friend and met his partner during his visit to the United States last month.

The highly symbolic gesture, indicative of what is seen as Francis's personally tolerant attitude towards gays, came a day before the pontiff met prominent gay marriage opponent Kim Davis -- another private encounter in the United States that the Vatican said did not indicate support for her stance.

At the heart of the synod's agenda is Francis's belief that the Church must demonstrate mercy in its bid to address the gulf between what it currently says about marriage, love and sex and what tens of millions of its followers actually do every day.

Francis appears favorable to change on this issue, saying the Church must tend to the "wounds" caused by family breakdown rather than judging followers in "irregular" situations.

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