This is where many of the factors I previously described come to a head. In particular, it draws out the conflict between God's promise and Sarai's barrenness. There is a tension here as well, as obviously everyone is thinking about the inheritance of this wealthy man. Will he have a child? Both Abram and Sarai are getting very old at this point, near or past the age of childbearing.
Sarai proposes that Abram have sex with Hagar, her maid, that through her maid she might have children. This is something we will see later in Genesis, but I don't know much about how historically common this is.
This chapter is very important biblically because it is also quoted in the NT (in Galatians) as a metaphor for two different groups of people, those who are children of the Flesh and follow the Law vs. those who are children of the Spirit who follow the Spirit.
One thing that is fascinating about this episode is that there is no part of the bible that explicitly judges it on moral grounds (whether to be the right thing to do or more likely, the wrong thing to do). This has some interesting results, because it has left the moral interpretation of the events largely open to anyone impressing their particular beliefs onto it. I don't think this is the right approach, but it is certainly the common approach.
The reason for leaving out moral judgment, in my opinion, is that the biblical author is presuming that the reader shares his moral viewpoint. This is really the best and most sensible rationale I can think of. I don't think it's for implicitly avowing moral relativism (i.e. polygamy is wrong for me, but it's right for you), and it's *not* because "anything that is not condemned is approved". This is a very common (implied) exegetical argument, that is never exactly stated in those words but is referenced all the time. That is, people use the non-condemnation of Abram's polygamy (and later, Jacob doing the same) either 1) as proof that polygamy is OK (this is less common), or 2) as proof that the bible is anachronistic, outdated and invalid to modern society (this is much, much more common). Naturally, I don't like either conclusion, and not just because I dislike the results. It's because the "means", the interpretative assumption, is incorrect.
This whole "if it's not condemned, then it's approved" line of reasoning is applied to a number of different passages, usually with the expressed purpose of showing that the bible approves of a number of different disagreeable actions by various actors, and then the usual conclusion is the irrelevancy of the bible. It can be difficult to argue against this reasoning because, well, the bible doesn't actually explicitly condemn those actions (at least, in the space of the relevant passage), so it can be difficult to show that the bible indeed implicitly condemns the actions. I will try to show my readers that the bible has a consistent pattern of implicitly condemning actions, and from there extend this principle to other areas such as here, in Gen 16. This is a really important issue because it comes up all the time in Christian apologetics, and it is very easy for people who are not familiar with the bible to get tripped up by these passages when they are stated without proper context.
I can think of several important "not explicitly condemned" actions in the bible that are clearly contrary to the stated ordinances of the Law, which is the basis of Hebrew morality. First, nearly the entire book of Judges. I won't give a detailed analysis here (the Judges section of my commentary should cover this), but I will list a few example verses: 14:2 (marrying outside of Israel), 14:9 (touching and eating from a dead wild animal, and it's even unclean), 17:3-5 (graven image, though arguably verse 6 is a condemnation), 18:27 (theft), 18:31 (graven image). The second example is 1 Samuel 3:3 (the lamp should never go out). I could probably find a few more if I looked carefully, but it's true that most instances of violations of the Law are condemned in some fashion. Having multiple wives is not condemned by the Law, but in a modern context that means very little to a Christian who has to consider the broad re-interpretation of the OT postulated by life in the New Covenant.
Either way, from a logical point of view, critics are actually trying to make an affirmative statement here (the bible supports [X] where [X] might be slavery, polygamy, etc). As such, it is necessary to offer evidence of the affirmative statement, and only listing verses where the practice is mentioned (e.g. Abram's polygamy) does not prove that it is positively supported by the bible. It only proves that it is mentioned in the bible. You might as well say that historians support the Holocaust because they discuss the Holocaust.
Honestly, I wouldn't spend so much time on this issue if I didn't see it raised so often by skeptics and others who are not really versed in biblical hermeneutics. As I've demonstrated, it is not logically sound and also, if applied consistently across the bible, produces logical contradictions (i.e. that the bible supports things it also condemns like touching or eating a dead wild animal). I will now return to the events of Gen 16.
So anyway, both Abram and Sarai are desperate to have a child, so they take to desperate measures. Abram, one can speculate, was not actually told that his son would also be Sarai's child. At least, it's not mentioned so far in the bible. So perhaps he thought that this action was in line with what God wanted him to do. Who knows.
It immediately states that Hagar conceived a child, and things go sour just as quickly. Hagar starts to get perhaps... pleased with her success where Sarai failed, Sarai gets immediately jealous and challenges Abram. Abram prioritizes Sarai over Hagar, and so Sarai starts to abuse Hagar. Hagar (still pregnant) promptly flees.
All of this strife shows that Abram's decision is quickly proving for the worse. Hagar and Sarai are immediately in conflict, and Sarai is angry with Abram even though he simply did what she proposed. So Sarai appears to be conflicted with herself, having suggested for Abram to do something and then getting angry with him when he does. To be humorous for a moment, "that's just like a woman." And of course, Abram is being just like a man. "But you told me to do it! How can you be angry?" But in this case, Abram partially makes up for this by caving to Sarai's pressure and allowing Sarai to dominate Hagar: Hagar was perhaps thinking that by bearing Abram's firstborn son, she would be elevated in power, maybe even above Sarai. But Abram proves his loyalty first to his wife, even though she was barren.
Obviously life would have been simpler, and perhaps easier, if he had never had sex with Hagar. This is another reason why some people think Abram makes a mistake here, because of the short-term consequences. We will see this strife continue in later chapters.
Interestingly, an angel appears to Hagar and tells her to go back. Even though Hagar (and Ishmael) are not in the genealogy of Christ or the Jewish people, they are still important to God so that he sends the "Angel of the LORD" (which is variously interpreted as either an angel, an important angel, or a pre-incarnation of Christ, a Christophany). In my opinion, this shows at the least that God values everyone, even in OT times.
Then Hagar names God, and it's one of my favorite names, The God who sees me.
We also now have an age for Abram: he is 86 years old. This is very old by our standards, though if we presume he will live as long as his recent ancestors, he will live for some time yet.