From the perspective of Jewish law and Jewish thought one of the main points of marriage is for the husband and wife to carry on a sexual relationship that will lead to the birth of children, fulfilling God’s command to Adam and Eve of peru u’revu – to go forth and multiply. Will a marriage that cannot lead to a sexual relationship, referred to by the Gemara as kiddushin she-lo nimseru le-bi’ah, have any validity? The Talmudic sages Rava and Abaye disagree on this point. Abaye believes that such kiddushin will lead to marriage; according to Rava such kiddushin will have no significance. Rava’s source is a teaching that he received from bar Ahina who pointed out that the Biblical passage describing marriage states clearly that a man takes a woman and sleeps with her (see 24:1), indicating that without the potential for a sexual relationship the marriage has no meaning.
In defining kiddushin she-lo nimseru le-bi’ah, Rashi seems to indicate that this is any situation where a sexual relationship cannot develop in this marriage. In truth, we have no sources that would preclude marriage in a situation where some physical obstacle will keep the husband and wife from engaging in sexual relations. Moreover, as many rishonim point out, if the obstacle keeping the couple from engaging in sexual relations derives from halakhic sources (e.g. when the marriage is forbidden by the Torah, like a kohen marrying a divorcee), then the kiddushin does take effect, the couple is just not allowed to live together as man and wife and must get divorced.
Because of these questions, most of the commentaries explain that kiddushin she-lo nimseru le-bi’ah means a case where the act of kiddushin itself creates a forbidden relationship that will not allow the husband and wife to live together. It is only that case where Rava would rule that the kiddushin cannot possibly be recognized as creating a marriage, if that marriage can never be consummated. Examples of such cases are discussed in the Gemara, e.g. when a man offers kesef kiddushin to two sisters simultaneously. Since a man cannot be married to two sisters, the kiddushin will have no meaning.