Brachos 61a deals with this subject. According to Rashi (see Maharsha), there are 2 issues. First, there is a hirhur problem. Additionally, the gemara extends the halacha to one's own wife. This, acc. to Rashi, is a "best practices" problem (ayin sham v'ayin maharsha). According to the Rambam (see Maharsha), the second issue may be a kedusha issue and merely an extension of the first issue.
The next halachos in the gemara, which are similar in nature to our subject, Rashi says specifically are talking about a married woman (who is not your wife-maharsha). This may be regarding the issur. The spirit of the halacha would apply to an unmarried woman depending on the circumstance (see S"A Eh"E 21:3).
When I was dating, I consulted my rebbi regarding opening the door for my date. My Rebbe was matir if the girl was not aware of the halacha (I stalled for a second after opening the door to verify). I don't know whether this heter would apply to an aishes ish, too. This subject would best be answered by your rav. Personally, when in this circumstance, I hold the door open, but do not walk in forward behind the woman (my practice depends on the circumstance).