Objection 1. It would seem that other sins besides unbelief dissolve marriage. For adultery is seemingly more directly opposed to marriage than unbelief is. But unbelief dissolves marriage in a certain case so that it is lawful to marry again. Therefore adultery has the same effect.
Objection 2. Further, just as unbelief is spiritual fornication, so is any kind of sin. If, then unbelief dissolves marriage because it is spiritual fornication, for the same reason any kind of sin will dissolve marriage.
Objection 3. Further, it is said (Matthew 5:30): "If thy right hand scandalize thee, pluck it off and cast it from thee," and a gloss of Jerome says that "by the hand and the right eye we may understand our brother, wife, relatives and children." Now these become obstacles to us by any kind of sin. Therefore marriage can be dissolved on account of any kind of sin.
Objection 4. Further, covetousness is idolatry according to Ephesians 5:5. Now a wife may be put away on account of idolatry. Therefore in like manner she can be put away on account of covetousness, as also on account of other sins graver than covetousness.
Objection 5. Further, the Master says this expressly (Sent. iv, D, 30).
On the contrary, It is said (Matthew 5:32): "Whosoever shall put away his wife, excepting for the cause of fornication, maketh her to commit adultery."
Further, if this were true, divorces would be made all day long, since it is rare to find a marriage wherein one of the parties does not fall into sin.
I answer that, Bodily fornication and unbelief have a special contrariety to the goods of marriage, as stated above (Article 3). Hence they are specially effective in dissolving marriages. Nevertheless it must be observed that marriage is dissolved in two ways. In one way as to the marriage tie, and thus marriage cannot be dissolved after it is ratified, neither by unbelief nor by adultery. But if it be not ratified, the tie is dissolved, if the one party remain in unbelief, and the other being converted to the faith has married again. On the other hand the aforesaid tie is not dissolved by adultery, else the unbeliever would be free to give a bill of divorce to his adulterous wife, and having put her away, could take another wife, which is false. In another way marriage is dissolved as to the act, and thus it can be dissolved on account of either unbelief or fornication. But marriage cannot be dissolved even as to the act on account of other sins, unless perchance the husband wish to cease from intercourse with his wife in order to punish her by depriving her of the comfort of his presence.
Reply to Objection 1. Although adultery is opposed to marriage as fulfilling an office of nature, more directly than unbelief, it is the other way about if we consider marriage as a sacrament of the Church, from which source it derives perfect stability, inasmuch as it signifies the indissoluble union of Christ with the Church. Wherefore the marriage that is not ratified can be dissolved as to the marriage tie on account of unbelief rather than on account of adultery.
Reply to Objection 2. The primal union of the soul to God is by faith, and consequently the soul is thereby espoused to God as it were, according to Hosea 2:20, "I will espouse thee to Me in faith." Hence in Holy Writ idolatry and unbelief are specially designated by the name of fornication: whereas other sins are called spiritual fornications by a more remote signification.
Reply to Objection 3. This applies to the case when the wife proves a notable occasion of sin to her husband, so that he has reason to fear his being in danger: for then the husband can withdraw from living with her, as stated above (Article 5).
Reply to Objection 4. Covetousness is said to be idolatry on account of a certain likeness of bondage, because both the covetous and the idolater serve the creature rather than the Creator; but not on account of likeness of unbelief, since unbelief corrupts the intellect whereas covetousness corrupts the affections.
Reply to Objection 5. The words of the Master refer to betrothal, because a betrothal can be rescinded on account of a subsequent crime. Or, if he is speaking of marriage, they must be referred to the severing of mutual companionship for a time, as stated above, or to the case when the wife is unwilling to cohabit except on the condition of sinning, for instance, if she were to say: "I will not remain your wife unless you amass wealth for me by theft," for then he ought to leave her rather than thieve.
Source: Summa Theologica (New Advent)