Genesis Chapter 16
Sarai had a maidservant named Hagar, and she told Abram to lay with Hagar so he could have children. Abram listened and had Hagar as a wife (vs. 3) and she conceived. As soon as Hagar conceived she looked at Sarai with contempt toward her. And Sarai blamed Abram. Abram told Sarai to deal with Hagar however she pleased, so she was harsh with her and Hagar fled.
The angel of the LORD found Hagar in the wilderness by a well (Beer-lahai-roi which means, "well of the Living One who sees me.") and told her to return to Sarai and He will surely multiply her offspring so that they cannot be numbered. The angel of the LORD told her to name her son Ishmael and that he would be a wild donkey of a man, and he will be against everyone. So Hagar bore Ishmael when Abram was 86.
Sarai and Abram appear to have good intentions here, but they are mixed with sinful actions. Marriage was set up to be between one man and one woman, and to lay with someone outside of marriage not only causes conflict between the women, but it is a false witness to the relationship God has between His people. There is only one God, no others. Therefore a marriage is with one other person, not many others. When it says, "Abram...had Hagar as a wife," it doesn't mean she was his wife, but that he behaved with her in a manner that a man should only behave with his wife. This distrust that God would fulfill his promise to give Abram children through His divine means led to sin, and then to a generation through Ishmael who caused strife and unbelief among the nations that we even feel the effects of to this day. After all, this is where Islam claims a heritage from. Islam falsely believes that Ishmael was God’s chosen line to bless the nations. This seemingly small decision has had ramifications on people throughout the history of the world, and until Jesus returns it probably will continue.