Mt. 19:9 "exceptive clause"

[Geremia]

In support of the interpretation that Matthew 5:32 or 19:9 can refer to concubinage is the fact that the word for adultery most commonly used in the OT and NT is μοιχεία, not πορνεία (cf. 1 Cor. 5:1).
Orchard et al., A Catholic Commentary on Holy Scripture:

An alternative Catholic explanation (recently revived, amended and given rabbinic background by J. Bonsirven, S.J., Le Divorce dans le Nouveau Testament, Tournai 1948) objects that the usual OT and NT word for 'adultery' is μοιχεία. It maintains that πορνεία here means concubinage (cf. 1 Cor 5:1)—incestuous marriage within the degree forbidden by the Mosaic Law, Lev 18:1–17. In such a case a man in dismissing (divorcing) the woman is not only guiltless but is actually doing his duty. Our Lord inserts the clause in order to hint that the Mosaic injunctions remain (cf. CR [Clergy Review] 20 [1941], 283–94).