Our Gemara presents the case of a man who appointed a messenger to arrange a marriage on his behalf. Rabbi Yohanan rules that if he gave no instructions, we must assume that his messenger did his bidding and that he is now married to someone. In the event that the messenger does not return and we do not know to whom the man is married, we are forced to conclude that the man cannot marry anyone, since she might be an immediate relative (a sister, daughter, mother, etc.) of the woman to whom he is married. Rava points out that he will be able to marry a woman who has absolutely no single female relatives, since we can be certain that there will be no forbidden relationships.
The question raised by the rishonim is whether this situation should affect other people as well. Perhaps we must suspect that every woman may be the unknown wife of the man in the original story – and now no one can get married! Even if we accept the word of the woman that she never accepted kiddushin from anyone, how can we be certain that her father did not accept kiddushin on her behalf when she was a child.
Several approaches are put forward to distinguish between the man who appointed the original messenger and all other men in the world.
In his Sefer HaYashar, Rabbeinu Tam explains that our assumption that a messenger always is assumed to fulfill his task only works on a Rabbinic level. Thus, on a biblical level the man really can marry whoever he wants. The ruling that forbids him to marry is a kenas – a penalty that the Sages placed on him for being so flippant about marriage that he sent a messenger out without specific instructions regarding who his wife was to be. Obviously this penalty applies only to the man and not to anyone else.
Another suggestion is that in this case specifically we cannot assume that the messenger fulfilled his mission because it is not totally in his hands – marriage can only be accomplished with the agreement of the woman, and we cannot be sure that he found a woman who agreed to the marriage. Thus we are in a situation of safek – of doubt – which will affect the man himself, but no one else.