When someone gets married, Ketubot 7b tells us that the following words appear in one of the blessings recited:
והתיר לנו את הנשואות על ידי חופה וקידושין [נ”א בקידושין]
These are the words recorded by Maimonides, Hilkhot Ishut 3:24, and are found in almost all early sources from the Sephardic world, as well as in many from the Ashkenazic world. However, at Ashkenazic weddings (and I believe at all Sephardic weddings as well, see R. Ovadiah Yosef, Yabia Omer, vol. 5, Even ha-Ezer no. 6) the following is recited:
והתיר לנו את הנשואות לנו על ידי חופה וקדושין
Why do we repeat the word לנו as it is not part of the original blessing? R. Nissim, in his commentary on the Rif to Ketubot (p. 2b in the Alfasi pages), writes:
ורבינו תם ז”ל הגיה והתיר לנו את הנשואות לנו
What this means is that R. Tam altered the text of a blessing that had been in use for probably a thousand years. This is obviously a very radical step, so what led him to do it? As R. Meir Mazuz, Or Torah, Av 5772, p. 1012, points out, the original formulation is somewhat ambiguous. It states והתיר לנו את הנשואות, which could be understood to mean that through this blessing one can now have sexual relations with all married women. (This reason for R. Tam’s emendation is also noted by the Taz, Even ha-Ezer 34:3, R. Samuel ha-Levi, Nahalat Shiv’ah [Bnei Brak, 2006], 12:6, and many others.) Ritva, Ketubot 7b, wonders why the blessing is so ambiguous:
ותימה גדולה למה תקנו לשון סתום בזה הברכה הנאמרת ברבים
Not noted by R. Mazuz is that Rashi is cognizant of the problem and on the words והתיר לנו explains:
את נשותינו הנשואות לנו על ידי חופה וקידושין
What R. Tam did was take the explanation of Rashi and insert it into the blessing by his addition of the word לנו. It is, of course, difficult to imagine that anyone would have really assumed that the blessing allowed him to sleep with other married women. Yet according to R. Tam, the fact that the words could be understood in an improper way was reason enough to alter them. R. Abraham ben Nathan notes that R. Tam’s emendation was directed towards the stupid people. See his commentary to Kallah Rabbati (Tiberias, 1906), p. 7:
ומנהג צרפת בברכה לומר והתיר לנו את הנשואות לנו, לרווחא דמילתא שלא להטעות הפתיים
Although by now R. Tam’s emendation is the standard version recited by Ashkenazim and Sephardim, neither R. Joseph Karo nor R. Moses Isserles, Shulhan Arukh, Even ha-Ezer 34:1, refer to it.
Marc B. Shapiro, “Assorted Comments”, The Seforim Blog (12 June 2014), n. 15 [http://seforim.blogspot.com/2014/06/assorted-comments.html]