The quieting of concupiscence (concupiscentiæ sedatio, as Pope Pius XI descriptively calls it in Casti Connubii) is a secondary end of marriage.If that's their goal, then why marry? Such an end can be achieved safer and more readily in the religious than the married state.
The primary end of marriage is the procreation and education of children, not the sanctification of the spouses.

The Church always taught from the time of Christ that the primary purpose of the Holy Sacrament of Matrimony is to sanctify our lives.

Lust should have no part in our Christian lives, for one can lust within marriage and be damned.

The Church always taught from the time of Christ that the primary purpose of the Holy Sacrament of Matrimony is to sanctify our lives

and to raise our children in the Holy Faith so that they become saints.

Lust should have no part in our Christian lives, for one can lust within marriage and be damned.

Ego enim dico, uti libidine non semper esse peccatum; quia malo bene uti non est peccatum.
I hold that to use lust [libidine] is not always a sin, because to use evil well is not a sin.

Can you quote where the Church has taught this is marriage's primary purpose?

St. Thomas ranks Baptism as the most important sacrament. He ranks Matrimony last "because it has less participation in the nature of the spiritual life, to which the sacraments are ordained".

Thus, married Catholics are more sanctified by the top 5 sacraments (esp. Baptism and Eucharist) than by marriage.

(To enter marriage with the primary and sole purpose of sanctifying oneself and not to have children would render the marriage invalid.)

The post-Vatican II teachings that make mutual love of the spouses the primary end of marriage tend to glorify the sanctifying aspect of marriage to such an extent that they degrade the necessity of the other sacraments (which Paul VI bastardized anyways with his Modernist Novus Ordo new-sacraments). Sanctify-yourself-with-more-sex-and-fewer-sacraments is certainly more appealing to "modern man" than the Eucharist or Penance, so it makes sense why the cult of man Novus Ordo sect promotes abominations like "Theology of the Body".
Yes, that's marriage's primary end.
Sure, as in contraception, sodomitical acts, etc., but one can use the evil of concupiscence for a good purpose in marriage:
Contra Iulianum 5.60:(For more quotes along these lines, see §2. "Saint Augustine and 'Putting Bad to Good Use'" of this article)

In my case wifey doesn’t want any more kids at 45 yrs of age. So I’m stuck having infrequent relations and when it happens i have to stop so as not to complete the act.  I confessed the sin (priest said not mortal) but i can see it happening again .  Obvuously abstinence is the safest way to go but it’s tough when the wife still wants the affection. I’m in a tough spot