John Chrysostom

"For God called us not for uncleanness, but in sanctification." Because he had said "his brother," and had also added, that God is the avenger, showing that even if an unbeliever has suffered this, he who has done it shall suffer punishment, he says, it is not as avenging him that He punishes thee, but because thou hast insulted Himself. He Himself called thee, thou hast insulted Him who called thee.

Homily on 1 Thessalonians 5 ↗

Theophylact of Ohrid

After having said that you wrong your brother, and having pointed out that God will avenge, he now expands his thought, showing that even if an unbeliever suffered this, the guilty one will still bear punishment. For God will punish you not in avenging him, but for His own sake — He called you so that you would be pure, and you have insulted by your impurity the One who gave you the Holy Spirit. Therefore, whether you defile your married slave or a queen, the guilt is the same, for you insult the one God. …

Commentary on 1 Thessalonians ↗

Thomas Aquinas

One reason for this is that God has called us, and a second reason is that such actions are contrary to God's gifts to us. If the Lord calls you to one thing and you do something contrary, then punishment is due. So Paul points out that God has not called us for uncleanness. "As he chose us in him before the foundation of the world, that we should be holy and blameless before him" (Eph. 1:4). "Those whom he predestined he also called" (Rom. 8:30).

Commentary on 1 Thessalonians ↗