The Gemara in a number of places (Sotah 28a, 47a) describes this as a sin which would prevent the Sotah from being punished by the water she drinks, since only when her husband is "Menukeh me'Avon" (clean of sin) can the water punish her, and he is forbidden from having relations with her at this stage. This would imply that although this is a sin, it does not carry a punishment from Beis Din but only causes the water not to work.

However, if a person had relations with his wife after Kinuy and Stirah before witnesses, he indeed could receive Malkus, since he flagrantly violated the Torah's prohibition. This prohibition is learned (Yevamos 11a) from the verse "Acharei Asher Hutam'ah" which comes to include the case of a Sotah who has not yet drunk. This is the opinion of Tosfos and Rashba in Yevamos 11a, who maintain that even a Safek Sotah is viewed as Teme'ah and is forbidden from having relations with her husband.

However, Tosfos in Sotah 28a writes that in this case there is no Malkus and this is also the opinion of the Rambam (Hilchos Gerushin 11:14).

Rav Chaim of Brisk explains this dispute as follows:

The first opinion maintains that we must treat the Sotah as if she has already committed adultery. Thus, she is forbidden like any divorcee who has had relations with others and in such a case there is Malkus.

However, the second opinion maintains that she is still in a state of doubt and is not to be deemed an adulterer yet. For this reason, the Torah created a new Isur, known as Sotah, to include even one who is not an adulterer in the prohibition. This Isur is mentioned as an Aseh in the Torah (meaning that the Parshah of Sotah says what he should do and from this can be inferred what he should not do), and therefore there is no Malkus.

Yoel Domb