Objection 1. It would seem that slavery cannot supervene to marriage, by the husband selling himself to another as slave. Because what is done by fraud and to another's detriment should not hold. But a husband who sells himself for a slave, does so sometimes to cheat marriage, and at least to the detriment of his wife. Therefore such a sale should not hold as to the effect of slavery.
Objection 2. Further, two favorable things outweigh one that is not favorable. Now marriage and freedom are favorable things and are contrary to slavery, which in law is not a favorable thing. Therefore such a slavery ought to be entirely annulled in marriage.
Objection 3. Further, in marriage husband and wife are on a par with one another. Now the wife cannot surrender herself to be a slave without her husband's consent. Therefore neither can the husband without his wife's consent.
Objection 4. Further, in natural things that which hinders a thing being generated destroys it after it has been generated. Now bondage of the husband, if unknown to the wife, is an impediment to the act of marriage before it is performed. Therefore if it could supervene to marriage it would dissolve it; which is unreasonable.
On the contrary, Everyone can give another that which is his own. Now the husband is his own master since he is free. Therefore he can surrender his right to another.
Further, a slave can marry without his master's consent, as stated above (Article 2). Therefore a husband can in like manner subject himself to a master, without his wife's consent.
I answer that, A husband is subject to his wife in those things which pertain to the act of nature; in these things they are equal, and the subjection of slavery does not extend thereto. Wherefore the husband, without his wife's knowledge, can surrender himself to be another's slave. Nor does this result in a dissolution of the marriage, since no impediment supervening to marriage can dissolve it, as stated above (50, 1, ad 7).
Reply to Objection 1. The fraud can indeed hurt the person who has acted fraudulently, but it cannot be prejudicial to another person: wherefore if the husband, to cheat his wife, surrender himself to be another's slave, It will be to his own prejudice, through his losing the inestimable good of freedom; whereas this can nowise be prejudicial to the wife, and he is bound to pay her the debt when she asks, and to do all that marriage requires of him for he cannot be taken away from these obligations by his master's command.
Reply to Objection 2. In so far as slavery is opposed to marriage, marriage is prejudicial to slavery, since the slave is bound then to pay the debt to his wife, though his master be unwilling.
Reply to Objection 3. Although husband and wife are considered to be on a par in the marriage act and in things relating to nature, to which the condition of slavery does not extend, nevertheless as regards the management of the household, and other such additional matters the husband is the head of the wife and should correct her, and not "vice versa." Hence the wife cannot surrender herself to be a slave without her husband's consent.
Reply to Objection 4. This argument considers corruptible things; and yet even in these there are many obstacles to generation that are not capable of destroying what is already generated. But in things which have stability it is possible to have an impediment which prevents a certain thing from beginning to be, yet does not cause it to cease to be; as instanced by the rational soul. It is the same with marriage, which is a lasting tie so long as this life lasts.
Source: Summa Theologica (New Advent)