Yevamot, Chapter Eleven, Mishnah One
A man may not marry the relatives of his wife (her mother, her daughter, her sister and some others) and he may not marry his fathers wife or his brothers wife. Our mishnah discusses whether a man may marry the relatives of a woman with whom he, his father or his son has had intercourse but not married. It also deals with the question of whether or not it is prohibited to have intercourse (but not marriage) with the relatives of ones wife.
We should note that the mishnah refers to two categories of women: one who has been raped and one who has been seduced. This is the common rabbinic way of referring to sex out of the bonds of marriage. The rabbis of the mishnah did not have a single word for extra-marital sex, and therefore referred to it using these terms which were used in other contexts.
1) A man is permitted to marry [the relative] of a woman [whom he has] raped or seduced.
2) He who rapes or seduces [a relative] of his married wife, is guilty.
3) A man may marry a woman whom his father has raped or seduced or a woman whom his son has raped or seduced.
a) Rabbi Yehudah forbids [marriage] with a woman whom ones father has raped or seduced.
Section one: Although it is forbidden (according to halakhah it is impossible) to marry the relatives of ones wife, a man may marry the relatives of a woman with whom he has had intercourse. The Torah forbids the marriage of a womans relatives only if he has had betrothal with that woman.
Section two: In contrast, it is forbidden to have any type of sex, even extra-marital with the relatives of ones wife. Since the relationship was sanctified by marriage, her relatives are prohibited.
Section three: Although it is forbidden (and impossible) to marry ones fathers wife or ones sons wife, the prohibition does not include a woman who had sex out of the bonds of marriage with the father or son. R. Yehudah prohibits sex with a woman who had sex with ones father. In the talmud this is learned midrashically from Deuteronomy 23:1.