Theophylact of Ohrid

But from the beginning it was not so: God joins two persons in the bond of marriage so that they become one, even leaving their own parents. Note that the Lord says: God does not permit polygamy, so that one wife could be divorced and another taken, and then that one abandoned and another married. If this had been pleasing to God, He would have created one husband and many wives; but it did not happen so, rather "God made them male and female," so that they would be joined — one husband with one wife.

Commentary on Mark ↗

Augustine of Hippo

Behold the Jews are convinced out of the books of Moses, that a wife is not to be put away, while they fancied that in putting her away, they were doing the will of Moses. In like manner from this place, from the witness of Christ Himself, we know this, that God made and joined male and female, for denying which the Manichees are condemned, resisting now not the books of Moses, but the Gospel of Christ.

Catena Aurea by Aquinas ↗

Bede

From the beginning of creation, God made them male and female. This is written in the beginning of Genesis. By saying male and female, He showed that second marriages are to be avoided. For He did not say male and females, as was sought from a prior divorce, but male and female, so that they might be bound in the companionship of one spouse.

On the Gospel of Mark ↗