The 161st prohibition is that the Kohen gadol — and only he1 — is forbidden from marrying a widow.

The source of this commandment is G‑d's statement,2 "He must not marry a widow, a divorcee, a chalalah or a zonah."

The Torah repeats the prohibition regarding the divorcee, chalalah and zonah for a Kohen gadol [even though he, as a Kohen, is already prohibited by virtue of the previous prohibitions] to cover the case described in tractate Kiddushin. That is, if the same woman was a widow, divorcee, zonah and chalalah — a Kohen gadol who had relations with her would receive four sets of lashes, and a regular Kohen who had relations with her would receive three sets. The statement there is, " 'a widow, a divorcee, a chalalah or a zonah' — if these [disqualifications] happened in order, he is punished for each separately." They thereby explain that this refers to one woman [with all four disqualifications].

When the Sages said, "in order," they meant that these disqualifications occurred [to her] in the order given in the verse, i.e., first she was widowed; then [she remarried and was subsequently] divorced; then became a chalalah3; and then a zonah.4

The reason we are forced to say this [i.e., that the four disqualifications happened in this order], is because we wish to find a case where he receives four sets of lashes for one woman in one act of intercourse. There is, however, a principle that a single act can not entail more than one prohibition, unless it is an issur mosif,5 an issur kollel,6 or an issur bas achas,7 as we explained in our commentary on tractate K'risus.8 If the [four disqualifications] occur in this order, each will be an issur mosif, as we explained there.9

If, however, [the prohibitions were associated with] different women — i.e., he had relations with [four different women,] one widow, one chalalah, one zonah, and one divorcee — it is obvious that he would receive lashes for each separate case.10

However, someone might ask the following question: "since we have a principle that one does not receive [more than one set of] lashes for a 'collective prohibition,'11 why should he receive lashes for each one [of the four]? They are all included in one statement?!"

You should know that because of this question, the Torah repeats the prohibition regarding a divorcee, zonah, and chalalah when mentioning the Kohen gadol. This teaches us that regarding them, he is just like a regular Kohen, who receives lashes for each prohibition individually.

[And how do we know this itself — that they count separately for the regular Kohen? The proof is as follows:] the regular Kohen receives lashes for each separately because since one of them was singled out, we learn that all count separately. It is [singled out in] the statement,12 "[they shall not marry a woman who is a zonah or a chalalah;] they shall not marry a woman who has been divorced from her husband." Since the divorcee was singled out,13 and he is lashed separately for this, [we derive that] so too he is lashed separately for the zonah and separately for the chalalah.

This is the meaning of the statement of our Sages in tractate Kiddushin,14 "just as the divorcee is singled out from the chalalah and zonah for a regular Kohen, so too are they separated for the Kohen gadol." There it is also explained that if they were different women, he receives lashes for each one separately, whether or not they were in this order.

We have therefore explained that each of these counts as a separate mitzvah, and therefore one receives lashes for each one separately.

It is also explained there that the regular Kohen never receives lashes unless he marries her and has relations with her, as they said, "if he had relations,15 he receives lashes, if he did not have relations,16 he does not receive lashes. [How do we know this?] One statement explains the other:17 Why does it say, 'he must not marry'? In order that, 'he must not profane.' "18

These four mitzvos are fully explained in tractates Yevamos and Kiddushin.