The sad thing is that Abraham gave in after he must have resisted the temptation to take another wife for many years. It shows something about his faith as well as his love for Sarah (or Sarai, as she was then). Even if he did cave in eventually.
A man would often divorce his wife if she was barren, or take an extra wife, (often, like Hagar, a slave-wife, a concubine), as well as the first wife. Jacob had two "proper" wives and two concubines, and had children by all of them. Hannah, Samuel's mother, was apparently the most loved wife, but her husband took another wife, probably because Hannah had no children. (I'm rather amused at Elkanah's clumsy attempt to comfort her, "Am I not better to you than ten sons?" - typical man ) ;-)
Blessings
Jeannette