What is the "remedy of concupiscence"?

Question

The "quieting of concupiscence" (remedium concupiscentiae) is one of the secondary ends of the sacrament of matrimony (cf. Casti Connubii §59), but what exactly is it? How does it "quiet" or "remedy" concupiscence?

Answer 1

St. Thomas describes how it works in Summa Theologica suppl. q. 42 a. 3 (on how matrimony confers grace) ad 4:

A remedy can be employed against concupiscence in two ways.

1. on the part of concupiscence by repressing it in its root, and thus matrimony affords a remedy by the grace given therein.
2. on the part of its act, and this in two ways:
   
   1. by depriving the act to which concupiscence inclines of its outward shamefulness, and this is done by the marriage blessings which justify carnal concupiscence;
   
   2. by hindering the shameful act, which is done by the very nature of the act because concupiscence, being satisfied by the conjugal act, does not incline so much to other wickedness. For this reason the Apostle says (1 Cor. 7:9): "It is better to marry than to burn." For though the works congenial to concupiscence are in themselves of a nature to increase concupiscence,* yet in so far as they are directed according to reason they repress concupiscence, because like acts result in like dispositions and habits.
   _{*i.e., engaging in the act makes them want to engage in it more, like how eating makes one want to eat more (if not restrained by reason)}

Summary:

Matrimony confers graces to allow one to practice

1. continence (with the help of matrimonial grace).
2. the marriage act without sin, because of the
   
   1. marriage goods (sacrament, children, fidelity)
   
   2. directing the act according to reason,

which honestant the marriage act (justify and dignify it).

St. Augustine says married couples use evil well; they use the concupiscence in the act (an evil) for a good purpose.