Promiscuity, unfaithfulness and unprotected sex are an offence against the sanctity of life, according to the head of the Anglican Church in South Africa.

Archbishop Thabo Makgoba made the remarks in a "moral state of the nation address" delivered at an interfaith function in Johannesburg on Wednesday night, and released on Thursday.

Although he did not mention President Jacob Zuma, his comments come amid controversy over the news that Zuma has fathered his twentieth child with a woman who is not one of his wives.

Makgoba said that it would be easy just to preach "no sex outside marriage", which was what Christians and others upheld because it made for strong families and healthy societies.

"But, if my words are completely out of touch with how people actually live, then I risk being dismissed as irrelevant," he said.

"Let me rather put it this way: promiscuity, unfaithfulness, adultery, unprotected sex that risks spreading HIV or resulting in unwanted pregnancies and the appallingly high numbers of abortions that occur in our country - all of these are offences against the sanctity, the sacredness, of life.

"They are acts of emotional violence and physical peril, and demeaning to the human dignity of all involved."

He said that such conduct was damaging to those involved, to the stability of society, and to future generations.

Sex was wonderful, and was one of God's best gifts to humanity.

"But the greatest gifts are open to the worst abuses. Let us use the gift of sexuality wisely and well."

Makgoba listed Mahatma Gandhi's "seven deadly social sins", one of which, he said, was "pleasure without conscience".

"As the American satirist Felicia Lamport put it: vice is nice. But a little virtue won't hurt you."