I would hazard a guess that about 90 percent of Bible readers (even including seasoned preachers) are under the misconception that Abraham ‘tief a likkle piece from Hagar’ [had unlawful sex] with Sarah’s complicity! The puzzling thing with such Bible readers is that they either dismissed the added dimension of maligning God who blessed Ishmael the fruit of the sex act between Abraham and Hagar (Gen. 17:20) or that passing thought did not force them to re-read the text of Scripture more closely.
Hagar was Sarah’s maidservant whom she gave to Abraham as his 2nd wife. The text is Genesis 16.3 which reads in the New King James Version thus: “Then Sarai, Abram’s wife, took Hagar her maid, the Egyptian, and gave her to her husband Abram to be his wife, after Abram had dwelt ten years in the land of Canaan.” There is no sexual misconduct here at all. It is helpful to know too what Christian Egyptologist and biblical scholar Kenneth Kitchen says concerning this phenomenon: “Having a maidservant as a wife (with the 1st wife’s approval) was customary at the time [similarly Jacob in Gen. 30.1-4] This was normal and normative in the first half of the second millennium, when a wife might come with her own maidservant, precisely so to serve as an insurance policy against possible inability to have offspring.” (Kenneth Kitchen, On the Reliability of the Old Testament, 325.)