Theophylact of Ohrid

Understand this also, that "he that is joined unto the Lord is of one spirit with Him" (I Cor. 6:17) and a union takes place between the believer and Christ. For we have all become one body with Him and we are, each one of us, members of Christ. So indeed no one can separate such a union, as Paul said, Who can separate us from the love of Christ? For what God has joined together, nothing can separate, as Paul says, neither man nor any other created thing, not angels, principalities, or powers (See Rom. 8:38-39).

Commentary on Matthew ↗

Jerome

God has joined by making man and woman one flesh; this then man may not put asunder, but God only. Man puts asunder, when from desire of a second wife the first is put away; God puts asunder, who also had joined, when by consent for the service of God we so have our wives as though we had them not. (1 Cor. 7:29.)

Catena Aurea by Aquinas ↗

Thomas Aquinas

Therefore now they are not two, but one flesh. Then he draws the principal conclusion: what therefore God has joined together, let no man put asunder, because it was done by God's will. If it is from God, man cannot separate; because if God joined, let God separate. For separation can be made by God or by man: and this either for pleasure, or to have some other woman, and this does not avail; or by mutual consent, so that one may serve God more freely, and so it is from God.

Commentary on Matthew ↗

Bonaventure

That conjugal chastity is to be approved according to the evangelical law is shown by authority, as follows. Matthew 19, the Lord responding to the question put to him concerning marriage: Have you not read, that he who made men from the beginning made them male and female: and afterward: What God has joined together, let not man separate? …

Disputed Questions on Evangelical Perfection, Question 3 ↗