The need to define the act of bi’ah (sexual intercourse) is essential in order to know when that significant act has taken place in a number of halakhic settings. Our Gemara suggests that in the vast majority of cases, the definition is ha’arah – a simple act of touching (in Shmuel’s words, “a kiss”). Although the focus of the Gemara is on the male organ, it appears that the conclusion is clear: penetration – of even miniscule proportions – of the man into the woman.
The Gemara suggests that the source for this is the case of a shifha harufa (a designated maidservant – see 19:20), where the Torah specifies that the act is significant only if there is a complete act of sexual relations, implying that in other cases even much lesser sexual contact would be considered to be significant. [Rashi interprets the case of shifha harufa as one where a non-Jewish maidservant is engaged to a Jewish slave – a relationship clearly permitted by the Torah. Other rishonim argue that Rashi is quoting only the opinion of Rabbi Yishmael, but we follow Rabbi Akiva, who explains that the case is one where a non-Jewish maidservant was owned jointly by two people, one of whom set her free, thus creating a situation where she is half-slave and half-free.]
Be that as it may, the Gemara brings a series of cases, each of which must be compared to the other so that the rule of ha’arah can be applied to each one. Thus the Gemara derives that
forbidden sexual relations,
forbidden sexual relations between a kohen and someone who he cannot marry,
the case where a yevama (widow) marries an outsider before being freed by halitza,
the case of yibum when the mitzva is fulfilled, and
the case of normal marriage
will all be accomplished by means of a sexual act, even if that act is only ha’arah.
The last case – that of normal marriage – refers to a case where a couple gets married via an act of bi’ah. Our Gemara clearly teaches that, from a technical standpoint, this case is identical to the other ones. Once ha’arah takes place, a significant sexual act has occurred, and the couple has successfully wed. The rishonim are quick to point out that this conclusion stands in contradiction with the Gemara in 1Massekhet Kiddushin (10a) that deals with the same case. There the Gemara says that a person embarking on a sexual relationship has in mind a complete act of sexual intercourse, and ha’arah would not suffice. Several suggestions are raised to deal with this seeming contradiction. One approach is suggested by the Ri”f, who argues that we must distinguish between kiddushin – the first act of marriage – that demands a completed sexual act, and nisu’in – which completes the marriage ceremony, where ha’arah will suffice.