Passion (from Latin passio or "to suffer") means, in the broad sense, letting something happen to oneself without actively controlling it. So, immoderate "sexual passion" or "sexual arousal" would mean letting sexual desires overtake oneself without moderating or guiding them by reason; failure to do this is lust. Concupiscence is the technical term for, as Fr. Hardon, S.J., defines it, the "Insubordination of man's desires to the dictates of reason".
Within marriage, the evil of concupiscence in the marital act is "excused" or "justified" by its relation to a marriage good such as children or marital fidelity (cf. "Of the Marriage Goods," Summa Theologica suppl. q. 49).
St. Augustine writes:
So let good spouses use the evil of concupiscence well, just as a wise man uses an imprudent servant for good tasks.
I hold that to use lust is not always a sin, because to use evil well is not a sin.
As for the warfare experienced by chaste persons, whether celibate or married, we assert that there could have been no such thing [as lust] in paradise before sin. Marriage is still the same, but in begetting children nothing evil would then have been used; now the evil of concupiscence is used well.
This evil is used well by faithful spouses.
(For sources, see notes 23-24 of marriage expert's Msgr. Cormac Burke's excellent article "A Postscript to the Remedium Concupiscientiæ [Remedy of Concupiscence]," The Thomist 70 (2006): 481-536; full-texts of St. Augustine can be found here or in St. Augustine on Marriage and Sexuality)
Addressing the question of whether Adam & Eve had carnal intercourse before the Fall, St. Thomas Aquinas gives this objection (Summa Theologica I q. 98 a. 2
Objection 3: Further, in carnal intercourse, more than at any other time, man becomes like the beasts, on account of the vehement delight which he takes therein; whence continence is praiseworthy, whereby man refrains from such pleasures. But man is compared to beasts by reason of sin, according to Ps. 48:13: "Man, when he was in honor, did not understand; he is compared to senseless beasts, and is become like to them." Therefore, before sin, there would have been no such intercourse of man and woman.
to which he responds:
Reply to Objection 3: Beasts are without reason. In this way man becomes, as it were, like them in coition, because he cannot moderate by reason the delectation of coition and the fervor of concupiscence. In the state of innocence nothing of this kind would have happened that was not regulated by reason, not because delight of sense was less, as some say (rather indeed would sensible delight have been the greater in proportion to the greater purity of nature and the greater sensibility of the body), but because the force of concupiscence would not have so inordinately thrown itself into such pleasure, being curbed by reason, whose place it is not to lessen sensual pleasure, but to prevent the force of concupiscence from cleaving to it immoderately. By "immoderately" I mean going beyond the bounds of reason, as a sober person does not take less pleasure in food taken in moderation than the glutton, but his concupiscence lingers less in such pleasures. This is what Augustine means by the words quoted, which do not exclude intensity of pleasure from the state of innocence, but ardor of desire and restlessness of the mind. Therefore continence would not have been praiseworthy in the state of innocence, whereas it is praiseworthy in our present state, not because it removes fecundity, but because it excludes inordinate desire. In that state fecundity would have been without lust.