A positive commandment doesn’t override this prohibition TB Yevamot 20

Although a brother’s wife is an ’ervah (ערוה), prohibited from marrying the husband’s brothers, the Torah permits an exemption when the husband dies without children. The widow has a levirate marriage with a brother in order to perpetuate her dead husband’s name. We learned that the positive commandment of yibum overrides the prohibition of marrying one’s brother’s wife. This principle that a positive commandment overrides a prohibition (אָתֵי עֲשֵׂה וְדָחֵי לֹא תַעֲשֶׂה) is learned from the juxtaposition of two verses 

(סְמוּכִים ). (See TB Yevamot 4a)

 The Torah prohibits the High Priest from marrying any woman was not a virgin.  Daf TB Yevamot 20 wonders why the above principle is not put into play in the case of the High Priest and a widow who was only betrothed to a his dead brother.  This widow is a virgin because the marriage was never consummated; consequently she is childless. Why don’t we say that the positive mitzvah of yibum overrides the negative prohibition of marrying the betrothed dead brother’s wife?

The Gemara provides three answers to our question and then rejects them before coming up with the final reason. The first answer is we have a special derasha that prohibits the positive commandment to override the prohibition in this case. “Rav Giddel said that Rav said in response: The verse states: “His yevama shall ascend to the gate to the Elders and say: My brother-in-law refused to establish a name for his brother in Israel, he did not wish to consummate the levirate marriage” (Deuteronomy 25:7). As there is no need for the verse to state: “His yevama,” since it is clear to whom the verse refers and no new information is added by this word, what is the meaning when the verse states: “His yevama”? It comes to teach that there is one yevama who ascends for ḥalitza but may not ascend for levirate marriage, and her brother-in-law is not given a choice. Who is this? This is a woman with whom it is prohibited for her yavam to enter into levirate marriage, as he would be liable for the violation of a prohibition, and the positive mitzva of levirate marriage does not override the prohibition.” (Sefaria.org translation) This answer is rejected as just not true based on the following discussion in the Gemara.

Gemara tries to manufacture a positive commandment that would override our case is the second attempted answer, but all these positive commandments were all too generic. Consequently, this attempt to answer our question was also rejected.

The third answer was a rabbinic decree (גְּזֵרָה דרבנן) that prohibited the High Priest from marrying his dead brother’s betrothed wife. This answer is rejected because this case was not common enough to warrant a rabbinic decree.

Finally the fourth answer was accepted as the reason why we don’t say that the positive commandment of yibum overrides the negative prohibition for the High Priest to marry his betrothed dead brother’s wife. Although the first intercourse is the mitzvah, all subsequent intimacies are forbidden. As a safeguard the rabbis prohibited even the first intercourse.

“Rather, Rava said that it is necessary to reject the previous suggestion and to offer a different reason: The first act of intercourse is prohibited by rabbinic decree due to the likelihood of a second act of intercourse. Although intercourse the first time with the yevama is the fulfillment of a positive mitzva, which does override the prohibition, once the mitzva is fulfilled with that act there is no longer any positive mitzva involved. Afterward, this yevama becomes prohibited because there is no longer a positive mitzva to override the prohibition. Therefore, due to the possibility that one might engage in intercourse a second time with this woman, the Sages decreed that even the first act is prohibited.

“The Gemara comments: This is also taught in a baraita: If one of those yevamin who may not marry their yevama due to a prohibition engaged in intercourse with her, he acquired her with the first act of intercourse; however, it is prohibited to retain her for a second act of intercourse.”

Tosefot ד"ה אַטּוּ בִּיאָה שְׁנִיָּה explains the difference between a regular levirate marriage and our case. When comes to a regular levirate marriage, the mitzvah knocks out the prohibition a brother’s wife altogether because the Torah commands yibum. This is not the case we been discussing. Although the first intercourse may be permitted; however, the other prohibition that the High Priest must marry only a single woman who is a virgin remains in place. Consequently, the positive commandment does not override prohibition.