Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon LIII

Canon LIII.

Whereas the spiritual relationship is greater than fleshly affinity; and since it has come to our knowledge that in some places certain persons who become sponsors to children in holy salvation-bearing baptism, afterwards contract matrimony with their mothers (being widows), we decree that for the future nothing of this sort is to be done.  But if any, after the present canon, shall be observed to do this, they must, in the first place, desist from this unlawful marriage, and then be subjected to the penalties of fornicators.

Notes.

Ancient Epitome of Canon LIII.

Godfathers cannot be permitted to be married with the mother of their godchildren.  If any one is so joined, let him do penance after separation.

Johnson.

(Clergyman’s Vade Mecum.)

The imperial law forbade the adopter parent to marry his or her adopted son or daughter; for the godchild was thought a sort of an adopted child.  See Justin., Institut., Lib. I., Tit. x.

Van Espen however refers, and to my mind with greater truth, to Justinian’s law (xxvj of the Cod. de Nuptiis) which forbids the marriage of a man with his nurse or with whoever received him from the font, “because,” says the law, “nothing can so incite to parental affection, and therefore induce a just prohibition of marriage, than a bond of this sort by which, through God’s meditation, their souls are bound together.”