Objection 1. It would seem that a spell cannot be an impediment to marriage. For the spells in question are caused by the operation of demons. But the demons have no more power to prevent the marriage act than other bodily actions; and these they cannot prevent, for thus they would upset the whole world if they hindered eating and walking and the like. Therefore they cannot hinder marriage by spells.
Objection 2. Further, God's work is stronger than the devil's. But a spell is the work of the devil. Therefore it cannot hinder marriage which is the work of God.
Objection 3. Further, no impediment, unless it be perpetual, voids the marriage contract. But a spell cannot be a perpetual impediment, for since the devil has no power over others than sinners, the spell will be removed if the sin be cast out, or by another spell, or by the exorcisms of the Church which are employed for the repression of the demon's power. Therefore a spell cannot be an impediment to marriage.
Objection 4. Further, carnal copulation cannot be hindered, unless there be an impediment to the generative power which is its principle. But the generative power of one man is equally related to all women. Therefore a spell cannot be an impediment in respect of one woman without being so also in respect of all.
On the contrary, It is stated in the Decretals (XXXIII, qu. 1, cap. iv): "If by sorcerers or witches...," and further on, "if they be incurable, they must be separated."
Further, the demons' power is greater than man's: "There is no power upon earth that can be compared with him who was made to fear no one" (Job 41:24). Now through the action of man, a person may be rendered incapable of carnal copulation by some power or by castration; and this is an impediment to marriage. Therefore much more can this be done by the power of a demon.
I answer that, Some have asserted that witchcraft is nothing in the world but an imagining of men who ascribed to spells those natural effects the causes of which are hidden. But this is contrary to the authority of holy men who state that the demons have power over men's bodies and imaginations, when God allows them: wherefore by their means wizards can work certain signs. Now this opinion grows from the root of unbelief or incredulity, because they do not believe that demons exist save only in the imagination of the common people, who ascribe to the demon the terrors which a man conjures from his thoughts, and because, owing to a vivid imagination, certain shapes such as he has in his thoughts become apparent to the senses, and then he believes that he sees the demons. But such assertions are rejected by the true faith whereby we believe that angels fell from heaven, and that the demons exist, and that by reason of their subtle nature they are able to do many things which we cannot; and those who induce them to do such things are called wizards.
Wherefore others have maintained that witchcraft can set up an impediment to carnal copulation, but that no such impediment is perpetual: hence it does not void the marriage contract, and they say that the laws asserting this have been revoked. But this is contrary to actual facts and to the new legislation which agrees with the old.
We must therefore draw a distinction: for the inability to copulate caused by witchcraft is either perpetual and then it voids marriage, or it is not perpetual and then it does not void marriage. And in order to put this to practical proof the Church has fixed the space of three years in the same way as we have stated with regard to frigidity (1). There is, however this difference between a spell and frigidity, that a person who is impotent through frigidity is equally impotent in relation to one as to another, and consequently when the marriage is dissolved, he is not permitted to marry another woman. whereas through witchcraft a man may be rendered impotent in relation to one woman and not to another, and consequently when the Church adjudges the marriage to be dissolved, each party is permitted to seek another partner in marriage.
Reply to Objection 1. The first corruption of sin whereby man became the slave of the devil was transmitted to us by the act of the generative power, and for this reason God allows the devil to exercise his power of witchcraft in this act more than in others. Even so the power of witchcraft is made manifest in serpents more than in other animals according to Genesis 3, since the devil tempted the woman through a serpent.
Reply to Objection 2. God's work may be hindered by the devil's work with God's permission; not that the devil is stronger than God so as to destroy His works by violence.
Reply to Objection 3. Some spells are so perpetual that they can have no human remedy, although God might afford a remedy by coercing the demon, or the demon by desisting. For, as wizards themselves admit, it does not always follow that what was done by one kind of witchcraft can be destroyed by another kind, and even though it were possible to use witchcraft as a remedy, it would nevertheless be reckoned to be perpetual, since nowise ought one to invoke the demon's help by witchcraft. Again, if the devil has been given power over a person on account of sin, it does not follow that his power ceases with the sin, because the punishment sometimes continues after the fault has been removed. And again, the exorcisms of the Church do not always avail to repress the demons in all their molestations of the body, if God will it so, but they always avail against those assaults of the demons against which they are chiefly instituted.
Reply to Objection 4. Witchcraft sometimes causes an impediment in relation to all, sometimes in relation to one only: because the devil is a voluntary cause not acting from natural necessity. Moreover, the impediment resulting from witchcraft may result from an impression made by the demon on a man's imagination, whereby he is deprived of the concupiscence that moves him in regard to a particular woman and not to another.
Source: Summa Theologica (New Advent)