Sanhedrin 19A

ואין עשה דוחה לא תעשה ועשה. אלא מן האירוסין אמאי? יבא עשה וידחה לא תעשה!

and there is a principle that a positive mitzva by itself does not override both a prohibition and a positive mitzva. But as for the ruling that he does not consummate levirate marriage with a widow from betrothal, why not? The positive mitzva to consummate levirate marriage should come and override the prohibition.

גזירה ביאה ראשונה, אטו ביאה שניה.

The Talmud answers: The first act of sex is prohibited by rabbinic decree due to the likelihood of a second act of sex. Although the first act of sex would fulfill the positive mitzva of consummating levirate marriage, which would override the prohibition against a High Priest’s having sex with a widow, any further sex would not be in fulfillment of a mitzva, and would not override the prohibition. Therefore, due to the possibility that the High Priest and the yevama would have sex a second time, the rabbis decreed that even the first act is forbidden.

תניא נמי הכי: אם קדמו ובעלו ביאה ראשונה ⁠– קנו, ואסור לקיימן בביאה שניה.

The Talmud comments: This is also taught in a baraita: If the High Priest or one whose yevama is forbidden to him went ahead and engaged in a first act of sex with her, he acquired her as a wife, but it is prohibited to retain that woman as a wife for a second act of sex.

מת לו מת כו׳. תנו רבנן: ״ומן המקדש לא יצא״⁠ – לא יצא עמהן, אבל יוצא הוא אחריהן. כיצד? הן נכסין והוא נגלה, הן ניגלין והוא נכסה.

§ The Mishnah teaches with regard to the High Priest that if a relative of his died, he does not follow the bier carrying the corpse. A baraita states: The verse concerning the High Priest, which states: “And from the Temple he should not emerge” (Leviticus 21:12), means: He should not emerge with them as they escort the bier, but he emerges after them. How so? Once they are concealed from sight by turning onto another street, he is revealed on the street they departed, and when they are revealed, then he is concealed.