First, the Bible's teaching on monogamy is very clear. Jesus" teaching is simply a restatement and direct application of Genesis 2 in it's plainest, simplest meaning. Second, most of the Bible's presenation of polygamy, though usually lacking in direct moral criticism, still portrays polygamy as a deeply troubled and troubling institution. Lamech, the first polygamist, is presented as a violent, arrogant, vengeful person, and his multiple wives seem to be as much an expression of his arrogance as his violence is. Likewise, many of the deepest conflicts and struggles of faith in the patriarchal stories are rooted in polygamy and the practise of keeping concubines. Ishmael's birth, for example, could not have taken place had Abraham not participated in the ancient custom of concubinage, nor would Sarah have offered that alternative. This story actually stresses that the highest path of faith for Abraham and Sarah was strict monogamy, and God clearly assumes that all along. David and Solomon's marriages also underline the pain and tragedy generated by men marrying many wives. Absalom's revolt involves a humiliation of David by taking David's "harem," a humiliation that simply could not have occurred on such a scale had David maintained a monogamous life. Solomon's apostasy is explicitly tied to his taking of many wives and concubines.
The OT stories underline and portray dramatically the main problem with polygamy. Polygamy rejects the central fact of biblical marriage, namely, that it is an eternal, indissoluble bond between two persons that cannot be duplicated. Our total allegiance and love for one and only one God is lived out practically by a total allegiance and love with one and only one person. Humans always want to "keep their options open" rather than focus on keeping covenant, and monogamy is the human laboratory where we learn that all-exclusive allegiance to one person, come what may.