I can relate to Sarai. Infertility can make a girl absolutely wacko. She told her husband to go have sex with Hagar, and then...get this...BLAMED him when he did exactly that! Poor Abram must have been so confused. PMS goes back thousands of years. He wasn't lusting after Hagar; he just did it to please his crazed wife. Verse 2 says, "And Abram listened to the voice of Sarai." Just like Adam listened to Eve. We girls have so much power over our men. They really do listen to our voice. Lord, may I never use my voice to influence my husband to do wrong, but only encourage him to follow Your ways.
It wasn't God's plan for Abram to be mating with Hagar, but He nonetheless ministered to her and blessed her. AND He blessed Sarai and gave her a son, even after she totally blew it! He is always good.