Beitzah 37A

דאית ליה אשה ובנים.

because it is dealing with a case in which he already has a wife and children, so that he has already fulfilled the mitzva to be fruitful and multiply, and his betrothal of another woman is only an optional act.

לא חולצין ולא מיבמין. והא מצוה קא עביד! לא צריכא, דאיכא גדול, ומצוה בגדול ליבם.

§ Nor perform ḥalitza, nor perform levirate marriage: The Talmud asks: But doesn’t one perform a mitzva through these acts? Why are they categorized as optional? The Talmud answers: No, it is necessary for the Mishnah to categorize them as optional, as it is speaking of a case when there is an older brother. Since the general principle is that the preferable way to carry out the mitzva is for the oldest brother to perform levirate marriage, the performance of levirate marriage by a younger brother is classified as optional.

וכלהו טעמא מאי ⁠— גזרה שמא יכתוב.

The Talmud clarifies the reason for the prohibition against judging, betrothing, etc., on Shabbat and Festivals: And in all these cases, what is the reason they may not be performed? It is a decree lest one write down the proceedings of these acts in a document, such as the verdict of a judgment, the document of betrothal, a document testifying to the ḥalitza, or a marriage contract in the case of levirate marriage.

ואלו הן משום מצוה: לא מקדישין ולא מעריכין ולא מחרימין ⁠— גזרה משום מקח וממכר.

§ It was taught in the Mishnah: And the following are notable because of the full-fledged mitzva involved in them, yet are prohibited on Shabbat: One may not consecrate, nor take a valuation vow, nor consecrate objects for use by the priests or the Temple. The Talmud explains: All these cases are prohibited because of a decree due to their similarity to commerce. These acts, which all involve the transfer of ownership to the Temple treasury, resemble commerce, which is prohibited on a Festival.