Objection 1. It would seem that madness is not an impediment to marriage. For spiritual marriage which is contracted in Baptism is more excellent than carnal marriage. But mad persons can be baptized. Therefore they can also marry.
Objection 2. Further, frigidity is an impediment to marriage because it impedes carnal copulation, which is not impeded by madness. Therefore neither is marriage impeded thereby.
Objection 3. Further, marriage is not voided save by a perpetual impediment. But one cannot tell whether madness is a perpetual impediment. Therefore it does not void marriage.
Objection 4. Further, the impediments that hinder marriage are sufficiently contained in the verses given above (Article 50). But they contain no mention of madness. Therefore, etc.
On the contrary, Madness removes the use of reason more than error does. But error is an impediment to marriage. Therefore madness is also.
Further, mad persons are not fit for making contracts. But marriage is a contract. Therefore, etc.
I answer that, The madness is either previous or subsequent to marriage. If subsequent, it nowise voids the marriage, but if it be previous, then the mad person either has lucid intervals, or not. If he has, then although it is not safe for him to marry during that interval, since he would not know how to educate his children, yet if he marries, the marriage is valid. But if he has no lucid intervals, or marries outside a lucid interval, then, since there can be no consent without use of reason, the marriage will be invalid.
Reply to Objection 1. The use of reason is not necessary for Baptism as its cause, in which way it is necessary for matrimony. Hence the comparison fails. We have, however, spoken of the Baptism of mad persons (III, 68, 12).
Reply to Objection 2. Madness impedes marriage on the part of the latter's cause which is the consent, although not on the part of the act as frigidity does. Yet the Master treats of it together with frigidity, because both are defects of nature (Sent. iv, D, 34).
Reply to Objection 3. A passing impediment which hinders the cause of marriage, namely the consent, voids marriage altogether. But an impediment that hinders the act must needs be perpetual in order to void the marriage.
Reply to Objection 4. This impediment is reducible to error, since in either case there is lack of consent on the part of the reason.
Source: Summa Theologica (New Advent)