Sorry for the lateness of this post, but the SAB was down all day, and I just recently figured out how to get the info I needed for the post. Hopefully it's sufficient.
I don't know whether to chalk it up to chauvinism or simple lack of biological understanding, but yes, as the SAB says, "In the Bible, it is the women who are barren, never the men." Most likely, if a couple wasn't having children, it probably would have been assumed to be the wife's problem in those days, unless the man had something obviously wrong with him. In this particular case, while it seems possible that there was more going on here than Sarai's infertility, it is notable that Abram had no difficulties conceiving with at least two other women besides Sarai, namely Hagar and Keturah. Also, the specific language ("went in unto" rather than "knew" Hagar) used is suggestive that Hagar conceived the very first time.
It was apparently a practice in those days that if a couple wanted children but the wife was unable to conceive, they would find a surrogate mother, usually one of the wife's female servants. (It's not made clear whether the servant has to be willing or not. One would hope so, but I suspect it was not the case.) A pretty barbaric practice, surely, and in the few cases where the Bible mentions someone using it, it's usually a story given a negative tone. We see here that the end result of using Hagar in this fashion is that Hagar is unhappy, Sarai is unhappy and Abram is unhappy. Eventually the child becomes the ancestor of the Ishmaelites, known today as the Arabs, who from their beginnings until today have very seldom gotten along well with the Israelites. So, as I said before, polygamy is allowed, but never fully approved of by God.
(Another note about possible strong language in the original Hebrew: In verse 5, the word translated "bosom" is actually more like "lap", and may have some strong connotations.)
So Sarai gets angry that Hagar conceived so easily, no doubt due to jealousy on multiple levels. Abram decides to placate her by letting her be mean to Hagar, which she is in some unspecified way, perhaps physical abuse. So Hagar runs away, and eventually has the first run-in with an "angel" in the book of Genesis. Note that it's a common belief among Christians that the phrase "angel of the Lord" is an indication of a Christophany. Whoever this angel is, he suggests to Hagar that she return and submit to whatever she has to, and that in time, her own son will become very prosperous, although warlike. Somehow, this encounter leads to the well being named after the event as "the well of the living One who sees."
So, Ishmael was born, and was he Abram's first son or not? It seems that he is, but the Bible often says otherwise, that Isaac was Abram's first son, and even his only son. What does this mean? The fact is that while Ishmael's father was indeed Abram, Isaac was Abram's first legitimate son. Although Abram cared about and loved Ishmael, Isaac was the only son of his wife Sarai. In that sense, Isaac was the first. But what about the children he had through Keturah in chapter 25? Wasn't Abram married to her? Yes, but I would venture to guess that in every case Isaac is called Abram's "only son," it's referring to a time before Abram remarried. It certainly is the case in the verses cited by the SAB, but someone may know of others that I don't.