The letter "Sub Catholicae" to the Bishop of Tusculum, of the Legation of the Apostolic See among the Greeks, March 6, 1254
453 18. Moreover concerning fornication which an unmarried man commits with an unmarried woman, there must not be any doubt at all that it is a mortal sin, since the Apostle declares that "fornicators as adulterers are cast out from the kingdom of God" (1Co 6,9).
455 20. Because according to the Apostle "a woman if her husband is dead is freed from the law of her husband" so "that she has the free power of marrying whom she will in the Lord" (cf. Rom. Rm 7,2 1Co 7,39), let the Greeks in no measure reprehend second or third or even later marriages; nor should they condemn but rather approve them between persons who otherwise can licitly be united to one another in marriage. Priests, however, should not by any means bless those who marry a second time
Tags: fornication, right to marry after death of spouse