Here is how G.H. Joyce explains it: About a hundred years later (716 A.D.) we have an instruction given by Gregory II (715-731) to the legates whom he was that year sending to Bavaria. In this document, speaking of the relation of married persons to one another, he says: "The Apostle has said: ‘Art thou bound to a wife? Seek not to be loosed’ : so long, that is, as your wife lives, seek not to pass over to carnal intercourse with another.’ [Footnote: Addit. ad Leg. Baiuwariorum. [[Latin text of the above quotation.]] (M.G.H., Leges iii, 453.)]
[Note for CAF readers: Before the paragraph I just quoted, Fr. Joyce had just quoted more than a dozen other popes and Church Fathers against divorce+remarriage, so these quotes from Pope Gregory II come after a long string.]
Much difficulty has been felt about a subsequent letter of the same Pope of 22 November, 726, addressed to St. Boniface, in which he replies to a series of enquiries made by the latter. In answer to one of these he says:
“As regards your question what a husband is to do, if his wife has been attacked by illness, so that she is incapable of conjugal intercourse, it were best if he could continue as he is and practice self-restraint. But since this demands exceptional virtue, the man who cannot live in continence, had better marry. But let him not fail to furnish her with support, since she is kept from married life by sickness, not debarred from it by some abominable offence.” [Footnote: [[Latin text of the above.]] (M.G.H., Ep. iii, 276.)
This decision has given rise to an immense amount of comment from the twelfth century to the present day. Many authors (including Gratian [Footnote: c, 18, C. xxxii, 7.]) have asserted without hesitation that the Pope gave permission for divorce and remarriage, thereby acting ultra vires. This is hardly credible. It supposes that he decided in a sense not merely opposed to his own express directions, but to the principle maintained for centuries against immense difficulties by his predecessors as an integral part of the Christian revelation. Since we do not possess St. Boniface’s letter, the precise terms of the enquiry must be a matter of conjecture. It is reasonable to suppose that it concerned a marriage in which, previously to consummation, the bride had been attacked by a disease which made conjugal life impossible.
[Footnote: This is the solution given by Rolandus, C. xxxii, 5; Mihi vero authenticum videtur hoc decretum quod et cuique praesenti casu inspecto manifeste liquebit. Sponsus quidam sponsam propriam in domum suam traduxit; verum aegritudine praeventa sponso debitum reddere non valuit: ea igitur jugi aegritudine detenta sponsus continere nequit. Summa Rolandi (ed. Thaner), p. 181; also p. 186 in C. xxxii, 7. Both Freisen (Geschichte des canonischen Eherechts, p. 331) and Fahrner (Geschichte der Ehescheidung, p. 63) accept this as the only reasonable explanation. The dissolution of unconsummated marriage did not become part of the Church’s regular practice till later. It may be supposed that in this case the Pope acted on the ground of the practical impotency of the woman.]
[Continuation of the paragraph above the footnote:] This is no far-fetched supposition. In certain cases Germanic custom tolerated the marriage of children. Thus in the Lombard code the legal age for marriage was the completion of the fourteenth year for a boy and of the thirteenth for a girl. But it is expressly declared that if the mundoaldus of the girl be her father or brother, he may give her to whom he wishes and at any age. Another law lays down the conditions under which a girl of full age might take a child husband. [Footnote: Lex Langobardorum, Liutprand, 12, 129 (M.G.H., Leges iv, III, 161).] Where this custom prevailed many marriages must have remained unconsummated for years. And, as will appear later (infra, p. 430 sqq.), until marriage is consummated it lies within the power of the Church to grant a divorce, and permit the parties to take other partners.
A few years later (5 January, 747) Pope St. Zacharias wrote to Pepin, then mayor of the palace, the bishops and other magnates of the Franks, a letter in which at Pepin’s request he sets forth the Church’s doctrine on a variety of points. There is no qualification in his teaching on this subject. He simply cites the forty-seventh of the Apostolical Canons, and the decree of the council of Carthage of 407 A.D. [Footnote: M.G.H., Ep. iii, 480.] This seems to furnish yet a further reason for regarding it as improbable that Gregory II was unfaithful to the traditions of the Holy See.
Again in the year 754 A.D. Stephen III is equally explicit in a letter to the monks of a place named Brittannacus in the diocese of Noyon. He cites the letter of Innocent I to Exsuperius of Toulouse, and declares that all who divorce their wives and make new marriages are to be excluded from the communion of the Church. [Footnote: Steph. III ad monasterium Brittan., c. 5 (Mansi, t. xii, p. 559).] Similar pronouncements of the Popes in the ninth and following centuries will find mention later. Their natural place is in connection with the Carlovingian reform and the subsequent developments. Source: Joyce, George Hayward. (1948.) Christian Marriage. (2nd ed.) London, U.K.: Sheed and Ward. p. 332-334
I hope that helps. God bless!