I'm a Latter-day Saint, so to clearly express my point of view on the scriptural mandate for monogamy, I need to refer to the Book of Mormon, which I consider scripture of equal validity and value with the Bible. I believe the Bible and Book of Mormon work togther to explain the circumstances in which having more than one wife was justified. Abraham, Isaac, and Jacob all had multiple wives and they were justified. If I were to take a second wife today, I would not be justified and would be excommunicated from my church (if I insisted on maintaining the practice). Let me explain why. The Book of Mormon setting is about 500 years B.C. The people of God began on their own to have more than one wife. The B of M prophet Jacob said: "For behold thus saith the Lord: this people begin to wax strong in iniquity; they understand not the scriptures, for they seek to excuse themselves in committing whoredoms, because of the things which were written concerning David, and Solomon his son...Wherefore I the Lord God will not suffer that this people shall do like unto them of old...For there shall not any man among you have save it be one wife; and concubines ye shall have none; For I the Lord God delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts."
These people were chastised by a prophet for thinking that because someone else had had more than one wife, they could also. Now, a later verse in Jacob is instructive:
"For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people, otherwise they shall hearken unto these things". Here the Lord says he may command otherwise, if he desires to "raise up seed unto him". This is, however, the exception and not the rule.
There have been special times in the past where God commanded a man to take one or more additional wives. This was done by the spirit of prophesy, was the Lord's will, and was not based on lust or anything other than a desire to comply with the will of God. God wanted to "raise up seed" unto him through the loins of Abraham. He commanded Abraham to reproduce through different women to become the father of many nations.
Abraham was a good man and was justified in what he did, even commanded to do what he did. We are under no such divine mandate today, so it's one man and one woman in one marriage.