John Chrysostom

Then his narrative proceeds thus: "For Herod had laid hold on John, and bound him, and put him in prison, for Herodias' sake, his brother Philip's wife. For John said unto him, It is not lawful for thee to have her. And when he would have put him to death, he feared the people, because they counted him as a prophet." And wherefore doth he not address his discourse at all to her, but to the man? Because it depended more on him. But see how inoffensive he makes his accusation, as relating a history rather than bringing a charge.

Homily on the Gospel of Matthew 48 ↗

Jerome

For Herod had arrested John, bound him, and put him in prison because of Herodias, his brother Philip's wife. Because John had said to him, 'It is not lawful for you to have her.' The ancient history tells us that Philip, the son of Herod the Great (under whom the Lord fled to Egypt), the brother of this Herod under whom Christ suffered, had married Herodias, the daughter of King Aretas; but later, due to some disputes arising against his son-in-law, he took his daughter back and gave her in marriage to Herod, his enemy …

Commentary on Matthew ↗

Thomas Aquinas

For he said to Herod: it is not lawful for you to have her. It should be known that Antipater, the father of Herod the king, was a foreigner, but he was a proselyte, hence his sons were Jews. But it was commanded in the law that while a brother was living, another should not have his brother's wife; therefore John, as a zealot for the law, said it is not lawful for you to have her.

Commentary on Matthew ↗

Glossa Ordinaria

And perhaps he observed the Jewish Law, according to which John forbade him this adultery.

Catena Aurea by Aquinas ↗