Some hold that it [the phrase] is a prohibition against polygamy, i.e., married to one at a time. This argument is stronger than one might suspect from its near universal rejection. However, while polygamy was common in Judaism it was not common in Christianity, so it seems unlikely that Paul would have thought to prohibit something that rarely occurred.
Was polygamy "common in Judaism" at the time of Paul--or decades later, during the composition of the Pastorals? In "Marriage, Divorce, and Family and Second Temple Judaism," published in Families in Ancient Israel (Louisville: Westminster John Knox, 1997), John J. Collins presents a thorough overview of the evidence, noting that it is mentioned by Josephus, Justin Martyr, and the Mishnah; it was apparently practiced by elites such as Herod and his sons; and the recent discovery of the Bathaba archive suggests that it may have spread beyond the upper classes of society to members of the Palestinian bourgeois. Nevertheless, while the legal wranglings of Bathaba and her fellow wife over the property of their late husband provide important evidence that polygamy was not limited to eminent figures such as the Herodians, the correspondence "remains a single instance and does not warrant any generalization about the extent of polygamy in second temple Judaism" (p. 122), and Collins does not dramatically depart from the relative consensus "that monogamy was the norm throughout the second temple period" (ibid, citing S. Lowy's "The Extent of Jewish Polygamy in Tannaitic Times" ). The fact that Justin Martyr is clearly engaged in a polemical dialogue with Judaism, and that the Mishnah often discusses past practices in a seemingly contemporary fashion, would support such a position.