Objection 1. It would seem that husband and wife are not equal in the marriage act. For according to Augustine (Gen. ad lit. xii) the agent is more noble than the patient. But in the marriage act the husband is as agent and the wife as patient. Therefore they are not equal in that act.
Objection 2. Further, the wife is not bound to pay her husband the debt without being asked; whereas he is so bound, as stated above (1,2). Therefore they are not equal in the marriage act.
Objection 3. Further, the woman was made on the man's account in reference to marriage according to Genesis 2:18, "Let us make him a help like unto himself." But that on account of which another thing is, is always the principal. Therefore, etc.
Objection 4. Further, marriage is chiefly directed to the marriage act. But in marriage "the husband is the head of the wife" (Ephesians 5:23). Therefore they are not equal in the aforesaid act.
On the contrary, It is written (1 Corinthians 7:4): "The husband... hath not power of his own body," and the same is said of the wife. Therefore they are equal in the marriage act.
Further, Marriage is a relation of equiparence, since it is a kind of union, as stated above (44, 1,3). Therefore husband and wife are equal in the marriage act.
I answer that, Equality is twofold, of quantity and of proportion. Equality of quantity is that which is observed between two quantities of the same measure, for instance a thing two cubits long and another two cubits in length. But equality of proportion is that which is observed between two proportions of the same kind as double to double. Accordingly, speaking of the first equality, husband and wife are not equal in marriage; neither as regards the marriage act, wherein the more noble part is due to the husband, nor as regards the household management, wherein the wife is ruled and the husband rules. But with reference to the second kind of equality, they are equal in both matters, because just as in both the marriage act and in the management of the household the husband is bound to the wife in all things pertaining to the husband, so is the wife bound to the husband in all things pertaining to the wife. It is in this sense that it is stated in the text (Sent. iv, D, 32) that they are equal in paying and demanding the debt.
Reply to Objection 1. Although it is more noble to be active than passive, there is the same proportion between patient and passivity as between agent and activity; and accordingly there is equality of proportion between them.
Reply to Objection 2. This is accidental. For the husband having the more noble part in the marriage act, it is natural that he should be less ashamed than the wife to ask for the debt. Hence it is that the wife is not bound to pay the debt to her husband without being asked, whereas the husband is bound to pay it to the wife.
Reply to Objection 3. This proves that they are not equal absolutely, but not that they are not equal in proportion.
Reply to Objection 4. Although the head is the principal member, yet just as the members are bound to the head in their own respective capacities, so is the head in its own capacity bound to the members: and thus there is equality of proportion between them.
Source: Summa Theologica (New Advent)