Vatican City - Only heterosexual families can form a family, Pope Francis said on Saturday, when he also condemned abortion and hailed the "sanctity" of women who forgive cheating husbands.

"It is painful to say this today: People speak of varied families, of various kinds of family," but "the family [as] man and woman in the image of God is the only one," Francis said in unscripted remarks.

"Many women - but even men sometimes do it [with wives] - wait in silence, looking the other way, waiting for their husband to become faithful again," the pope was quoted as saying by ANSA news agency.

This, he suggested, is "the sanctity that forgives all out of love."

Speaking to the Forum delle Famiglie, an Italian lay movement representing Catholic families, the pope also evoked Nazi eugenics programmes to condemn the abortion of seriously ill foetuses.