Q. It is forbidden to walk behind a woman. Is it permitted to do so if it is one’s wife, and she is in her days of purity, since one is in any case permitted to entertain libidinous thoughts of her at that time of the month?
Can one walk next to a woman that is not his wife?

A. The Talmud (Berachos 61a) teaches that the restriction to walk behind a woman applies even to one’s wife, as was the case with Manoach. Rashi (ibid.) mentions that it is a “gnai” or an inappropriate act. Einaim Lamishpot (ibid.) explains that Rashi’s intention is that it is improper due to the “chashad” that the people may suspect him of improper behavior, since not all may recognize his wife. Therefore the restriction would apply even when his wife is tahor. A similar ruling is to be found in Avos D’Rabi Nossan (2: 2) and in Binyan Yehoshua (ibid.).
Shulchan Aruch (E.H. 21: 1) rules that in principle one may walk, at the same pace (isalkena letzdadim) next (but not close) to another woman, not only his wife. However, Horav Shlomo Miller’s Shlit”a opinion is that the interpretation of isalkena letzdadim does not mean to walk on her side as opposed to behind her, rather it means to pass her over and walk in front. However the Rov added, that common sense, tznius and decorum must rule, as it is not the same to take a stroll on the park, to the helping and protecting of someone vulnerable or frail, to reach home.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a