John Calvin on Genesis 16:3

3. And gave her to her husband Abram to be his wife . Moses states what was the design of Sarai; for neither did she intend to make her house a brothel, nor to be the betrayer of her maid’s chastity, nor a pander for her husband. Yet Hagar is improperly called a wife; because she was brought into another person’s bed, against the law of God. Wherefore, let us know that this connection was so far illicit, as to be something between fornication and marriage. The same thing takes place with all those inventions which are appended to the word of God. For with whatever fair pretext they may be covered, there is an inherent corruption, which degenerates from the purity of the word, and vitiates the whole.