According to Catholicism, why are all sins against the 6th and 9th Commandments grave matter ex toto genere suo?

In my prayer book from 1930, it says, in an examination of conscience, under the 6th and 9th Commandment (emphasis mine): > It is sufficient to remind penitents that each and every act, if deliberate, contrary to the holy virtue of Purity--be it in thought or desire, in look, gesture, word, or deed--**is a Mortal Sin**, and as such must be mentioned in Confession intelligibly, yet modestly 6th and 9th commandment(pg 20) >6. Thou shalt not commit adultery. >9. Thou shalt not covet thy neighbor’s wife. This isn't the first time I've heard this, but this is the first time I can pinpoint to some kind of source. This specific prayer book, *A Manual of Prayers for the Use of the Catholic Laity * (pg 280) I should note was "PREPARED AND PUBLISHED BY ORDER OF THE THIRD PLENARY COUNCIL OF BALTIMORE." As is stated in all capitals on the title page. So it is not merely one theologian's opinion that informs the pages. So, **why is it that all sins against the 6th and 9th Commandments are gravely sinful?**