- This verse, which prohibits intercourse with a niddah, applies not just to one's wife but to any other women as well, married or not (Responsa Rivash 425, Leviticus 18:19)
- The Rabbis extend this prohibition to include not just sex, but all touching. And since unmarried women do not go to the mikveh, they are considered to be always in a state of niddah, and therefore are always off-limits for sex, or physical contact with men
רמב"ן:
מעשים כגון חיבוקים ונישוקים אינם מפרים מצוות לא יעשה של התורה , אלא איסור דרבנן בלבד
Ramban:
Acts such as hugging and kissing do not violate a negative commandment of the torah, but only a rabbinic prohibition
רמב"ם:
כל מי שנוגע אישה נידה בחיבה או רצון , אף אם המעשה נופל קצר של יחסי מין, מפר מצווה התורה שלילית
Rambam:
whoever touches a woman in niddah with affection or desire, even if the act falls short of intercourse, violates a negative torah commandment
- The Siftei Kohen further explains the Rambam by stating that the Rambam was only referring to hugging and kissing associated with intercourse. There are several places in the Talmud that the Amoraim hug and kiss their daughters and sisters and their behavior in considered permissible.
Sitting on a bus:
Rabbi Moshe Feinstein looked at the issue of public transportation and said that "regarding the permissibility of traveling in crowded busses and subways during rush hour, when it is difficult to avoid being pushed by women: Such physical contact involves no prohibition, because it does not contain any element of lust or desire"
- If it is not a lustful or desirable act, one is not held accountable for unintentional touching
Hand Shaking:
Rabbi Yehuda Henkin who is an orthodox Rabbi, explains that handshaking is not counted as an activity that can lead to sexual relations, so it's okay.
Shulchan Aruch forbids many types of interactions such as winks and gestures, but touching without intention of affect is not one of them.