Ninthly (v. 18), “You shall not bring the hire of a harlot, or the wages of a dog.” A remarkable law. The first part, concerning the hire of a harlot, can easily be understood to mean that public disrespect should be abominated. For God hates robbery as a burnt offering (Is. 61:8), hence also everything that has been gained by sin and wickedness; He wants to be worshiped reverently. Therefore nothing is to be brought to the house of the Lord, either for a sacrifice or for building it, which has the association of meanness and shame or depravity, lest God also be held in slight esteem when He lets Himself be worshiped with such mean and shameful things; for people easily tend to despise those whom they see pleased by contemptible things. He wants the common crowd to be seized and held in fear and reverence through an outward form. This is also the reason why the money for which a dog is bought should not be offered up, because the dog is regarded as the lowest animal in Scripture, one who serves even for proverbs, as Abner says (2 Sam. 3:8): “Am I a dog’s head?” He could not refer to anything lower. Whoever, therefore, honors God with the price of a dog, despises God and deems Him mean, as if he were offering up the price of offal. These signs reveal that the heart has a low opinion of God.
Luther’s works, vol. 9: Lectures on Deuteronomy 1525