Yevamot

— This  stands to reason, since betrothal of those forbidden by a negative precept is valid while the betrothal of those subject to kareth is not valid. Raba raised an objection: In the case of one forbidden by virtue of a commandment or by virtue of holiness, with whom the Ievir bad intercourse or participated in halizah, her rival is thereby exempt. Now, if one is to assume that those forbidden by a negative precept are Pentateuchally subject to halizah but not to the levirate marriage, why should her rival be exempt when he had intercourse with her? He raised the objection and he also supplied the answer: This is to be understood respectively;  'he had intercourse with her' refers to one prohibited by virtue of a commandment,  'participated in halizah with her' refers to the one forbidden by virtue of holiness. Raba raised an objection: He who is wounded in the stones or has his privy member cut off, a man-made saris,  and an old man, may either participate in halizah or contract levirate marriage. How?  If these died and were survived by brothers and by wives, and those brothers arose and addressed a ma'amar to the widows, or gave them letters of divorce, or participated with them in halizah, their actions are legally valid;  if they had intercourse with them, the widows become their lawful wives.  If the brothers died and they  arose and addressed a ma'amar to their wives, or gave them divorce, or participated with them in halizah, their actions are valid,  and if they had intercourse with them, the widows become their lawful wives but they  may not retain them, because it is said in the Scriptures — He that is wounded in the stones or hath his privy member cut off shall not enter [into the assembly of the Lord].  Now, if it could be assumed that those forbidden by a negative precept are Pentateuchally subject to halizah and not to levirate marriage, why should the widows become their lawful wives if they  had intercourse with them? But, said Raba, [say rather that] an erusin widow is forbidden  by both a positive and a negative precept, for it is written in the Scriptures, They shall be holy unto their God.  What, however, can be said in respect of a bastard or a nethinah?  — It is written, And sanctify yourselves.  If so,  all the [negative precepts of the] Torah should be regarded as positive and negative since it is written in the Scriptures, And sanctify yourselves!  But, said Raba, [the fact is that]  an erusin widow is forbidden  as a preventive measure against the marriage of a nissu'in widow.  What, however, can be replied in respect of a bastard and a nethinah?  — [The prohibition in] the case where a precept is applicable  is a preventive measure against [a marriage] where no precept is applicable. If so, let one's paternal brother's wife not be allowed levirate marriage as a preventive measure against marriage with the wife of his maternal brother! — 'We All Merciful made levirate marriage dependent on inheritance  [and the relationship] is, therefore, well known.  A woman, then, who has no children should not be taken in levirate marriage as a preventive measure against the marriage of a woman who has children! — The All Merciful made levirate marriage dependent on [the absence of] children, [and the fact  would be] well known. The wife of one's contemporary brother should not be taken in levirate marriage as a preventive measure against marriage with the wife of one's brother who was not one's contemporary! — The All Merciful has made it  dependent on dwelling together  [and the fact]  is well known. All women should not be taken in levirate marriage as a preventive measure against the marriage of a woman incapable of procreation! — This  is unusual.  A bastard and a nethinah also are unusual!  — But, said Raba, [this is the reason]: The first act of Intercourse  is forbidden  as a preventive measure against a second act of intercourse. It has been taught likewise: If they  had intercourse [with any of the forbidden women] they acquire [her as wife] by the first act of intercourse, but may not keep her for a second act of intercourse. Subsequently Raba, others say R. Ashi, said: The statement I made  is valueless,  for Resh Lakish said, 'Wherever you come upon a combination of a positive and a negative precept and  you are able to act in conformity with both, well and good; but if not, the positive precept must override the negative'.  Similarly here  it is possible to perform halizah, whereby one is enabled to keep the positive as well as the negative precept. An objection was raised: If they  had intercourse [with any of the forbidden women] they acquire [her as wife]!  — This is indeed a refutation. It was stated: Concerning an act of intercourse between a High Priest and a widow  [there is a difference of opinion between] R. Johanan and R. Eleazar. One maintains that it does not exempt her rival,  and the other maintains that it does exempt her rival.  )