John Calvin Commentary Genesis 16:3

"And Sarai, Abram`s wife, took Hagar the Egyptian, her handmaid, after Abram had dwelt ten years in the land of Canaan, and gave her to Abram her husband to be his wife." — Genesis 16:3 (ASV)

And gave her to her husband Abram to be his wife. Moses states what Sarai intended, for she intended neither to make her house a brothel, nor to betray her maidservant's chastity, nor to act as a panderer for her husband. Yet Hagar is improperly called a wife, because she was brought into another person’s bed, against the law of God.

Therefore, let us understand that this union was so illicit that it was something between fornication and marriage. The same thing occurs with all those inventions that are added to the word of God. For no matter what plausible pretext may cover them, there is an inherent corruption that deviates from the purity of the word and corrupts the whole.