We are learning the twelfth Mishnah of the fourth chapter of Maseches Kiddushin. This Mishnah deals with the laws of yichud - the prohibition against a man and a woman being secluded together.
"Lo yisyached adam im shtei nashim":
A man may not be secluded even with two women, and even with upright women - Jewish women who keep the mitzvos. The reason is the concern that he will entice one of them and be intimate with her, and the second will cover for her. Since there is a concern that something like this could happen, they may not be secluded together.
"Aval ishah achas misyachedes im shnei anashim":
One woman may be secluded with two men, provided that they are upright Jews who keep the mitzvos, because each of the men is embarrassed in front of his fellow. Such is the nature of men, that they do not cover for one another the way two women might.
Rabbi Shimon's view:
"Rabbi Shimon omer: af ish echad misyached im shtei nashim, bizman she'ishto imo, v'yashen imahem ba'pundaki, mipnei she'ishto meshamarto" - there are two ways to understand these words:
The first approach: Everything Rabbi Shimon says is one single statement - one man may be secluded with two women specifically when his wife is with him, and then he may even sleep with them in an inn, because his wife guards him and there is no concern here. According to this reading, the implication is that with one woman it would not be permitted. However, some commentators explain that the phrase "two women" is not meant precisely, and was used only because that is how the opening clause of the Mishnah was phrased; in truth it would be permitted even with one woman, since his wife is with him.
The second approach: Some commentators had a different version of the text of the Mishnah, and in their view Rabbi Shimon's words divide into two separate statements. "Af ish echad misyached im shtei nashim" - in this Rabbi Shimon disagrees with the first Tanna: the first Tanna said that two men may be with one woman, but one man may not be with two women, whereas Rabbi Shimon holds that just as two men may be secluded with one woman, so too two women may be secluded with one man, for they too will be embarrassed in front of each other. The continuation - "bizman she'ishto imo, v'yashen imahem ba'pundaki, mipnei she'ishto meshamarto" - is not Rabbi Shimon's statement but an anonymous Mishnah, and is the opinion of all.
"Misyached adam im imo v'im bito":
A man is permitted to be secluded with his mother or with his daughter, since there is no concern that anything will occur in a relationship of this sort, which has the potential of incest; we do not suspect such a thing, and people do not stumble in a matter like this. From here the Mishnah continues and gives details:
"V'yashen imahem b'kiruv basar" - they may even sleep together without clothing, and even with flesh touching flesh.
"V'im higdilu - zo yesheinah bik'susah v'zeh yashen bik'suso" - once the son or the daughter has grown up, each of them must be in his or her own clothing. They may share a bed, but each one in his or her own garment.
Some of the commentators wrote that the age referred to here is thirteen for a boy and twelve for a girl. The Gemara, however, says that if they have reached the point where the daughter is embarrassed to stand without her clothing in front of her father, then once she has reached that age - even if it is much younger than twelve - that is enough, and she is considered grown up for this purpose.
In summary: In this Mishnah we learned the laws of yichud - that a man may not be secluded with two women, while one woman may be secluded with two upright men; Rabbi Shimon's view and the two ways of understanding it; and the permission to be secluded with one's mother and with one's daughter, including the ruling that once they have grown up, she sleeps in her garment and he sleeps in his.