Abraham's Faithfulness?

Abraham married another wife, whose name was Keturah. [Genesis 25.1] 

Why?

We are left to our own conclusions as to why Abraham took on yet another wife and had children with her. However, if Hagar's son, Ishmael, was clearly not the child of promise, then Keturah's offspring were not children of promise either.

A simple search  in the Bible for one of Keturah's children, Midian, will reveal a group of people mentioned many times in the Bible known for their flagrant idolatry. Both Balaam's and Gideon's stories are based on Israel's conflict with the people of Midian.

If God told Abraham that Isaac was the child of promise and that all His blessings would come to and through Isaac, we must understand that Abraham's union with Keturah was just as faithless a move as was his hook-up with Hagar.

Romans 4 gives us quite a discussion about Abraham and his faith. The apostle Paul, in Romans 4, makes no mention of Abraham's missteps with other wives and other children (not children of promise), and yet, it uses pretty strong language praising Abraham's faithfulness:

Abraham never wavered in believing God’s promise. In fact, his faith grew stronger, and in this he brought glory to God. [Romans 4.20]

So, if Abraham never wavered in believing God’s promise, then what are Ishmael and Midian about (not to mention the other children)?

Father, I am at a loss for understanding Abraham's "faithfulness." However, I am certain that You require undivided faith in You alone. Perhaps an understanding of Abraham's faith contrasted with his relationships with Hagar and Keturah should be considered as Romans chapters 5 through 8 are read. Help me be strengthened by Abraham's story of faith and not deterred by his stories otherwise... So be it.