Pope Francis would seem to be referring to cases where there is grave matter, but not the other two criteria for mortal sin
While he acknowledges (along with Gaudium et Spes 51) the concern of some that a lack of sexual intimacy will lead to unfaithfulness, the pope also tells us that celibacy is what “the Church offers” to those in irregular unions who cannot live apart, and that “many” such couples “know and accept” it. Because he mentions this requirement here, it must necessarily be one of the criteria for the “certain cases” the pope has in mind in footnote 351, when the Eucharist may be given. Footnote 351 cannot be read in isolation, either from the text that precedes it or what the pope says elsewhere in Amoris. To do so would be to misread the note.
Still and all, it would have helped, in a document of this length, if the Holy Father had emphasized the positive overall effect of seeking an annulment where possible. That was a missed opportunity to talk about the healing nature of the process and to reemphasize that if the prior union was invalid, every other point is moot.
Scott Eric Alt is a freelance writer and Catholic convert who has been writing about apologetics and the Church for the past three years. He blogs at Patheos and also contributes to the National Catholic Register, Catholic Stand and Epic Pew.