Abram and Sarai dealt with the stress of Sarai not being able to have children and being too old to give birth.
According to the custom in that culture, a woman unable to have children could give her servant as a secondary wife to her husband. The child from this union became the primary wife's child. They adopted the child as the heir.
Sarai's suggestion was acceptable according to the culture's custom. But it was a human plan that showed a lack of faith in God. God’s promises would be fulfilled by faith.