When one reads the Bible, it is possible to understand it, not in terms of the ancient culture about which it was written, but in terms of one's modern experience. Such a case is found with the translation of mnēsteuō as engaged. In modern English, an engaged person has committed to become married, but is not yet married. Thus when reading such translations as the NRSV, NASB (1995ed), or CSB in Luke 2:5, one comes away with the idea that Mary was not yet Joseph's wife. The NIV translation "pledged to be married to him" and TEV/GNT translation "promised in marriage to him" give the same idea. The RSV, ESV, NKJV, and NASV (2020ed) use the word "betrothed," but the ESV note "one legally pledged to be married" reflects an English idea of betrothal, not a Jewish one.
A Jewish marriage in biblical times consisted of two stages: the betrothal and the taking. The former involved gifts that were given to the bride and her family (Gen. 24:53; 29:20), sometimes called bride-price or bride-wealth, although this is different from buying a slave wife (Ex. 21:7-11). The latter stage involved taking the wife to one's home (Matt. 1:24) and was often celebrated with a wedding ceremony(Gen. 29:27; Judg. 14:10,17; John 2:1ff.) and followed by consummation of the marriage (Gen. 29:21-23). But when the betrothal had taken place, the betrothed woman was considered a wife. If she had sex with another man, it was considered adultery and punishable by death (Deut. 22:23-24). Even before the taking of the wife, the marriage could only be dissolved by divorce (Matt. 1:19). The wife was considered betrothed until the marriage was consummated.
This is confirmed by the Jewish Encyclopedia online which says, "when the agreement had been entered into, it was definite and binding upon both groom and bride, who were considered as man and wife in all legal and religious aspects, except that of actual cohabitation. . . . . In strict accordance with this sense the rabbinical law declares that the betrothal is equivalent to an actual marriage and only to be dissolved by a formal divorce." (https://jewishencyclopedia.com/articles/3229-betrothal).
This explains why Mary who had been taken by Joseph as commanded to by the angel (Matt. 1:24) and was traveling with Joseph to Bethlehem (Luke 2:4-5) is still called his betrothed. She was his taken wife, but they had not consummated the marriage. Matthew says that Joseph did not start "knowing" her (that is, start having sex with her [an imperfect tense implying continuous action]) until she had given birth to her son Jesus (Matt. 1:25). So she was married at the time she gave birth to Jesus. Thus the KJV translates mnēsteuō as "espoused," meaning married, but that word is no longer used in common speech. Perhaps the best translation is "betrothed" with a note explaining the Jewish custom.
Bruce Terry