Josh, Baruch she'Kivanta to the Kashya of the Aruch la'Ner, who asks why the Gemara uses the plural word "Chayavei" if the only example is Be'ulah!

1) He answers based on the words of Tosfos (77b, DH Mechalel) in the name of Ri, the son of Rabeinu Meir, who writes that if a Kohen marries a Chalalah who was born through a transgression of a Chiyuv Aseh, this Chalalah does not represent a Lav prohibition to the Kohen but rather an Aseh. It seems that Tosfos learns that if a Kohen Gadol marries a Be'ulah, the offspring is a Chalalah; but even though, generally speaking, a Chalalah represents a Lav prohibition to a Kohen (as stated explicilty in the Torah, in Vayikra 21:7), nevertheless if the origin of the Chalalah daughter is from a mother who only represents an Aseh prohibition to the Kohen, this cannot generate a more severe prohibition -- namely, a Chalalah which is a Lav; it can generate only a Chalalah which is an Aseh.

2) The Aruch la'Ner cites the Mishneh l'Melech (Hilchos Isurei Bi'ah 19:3) who is in doubt about the Din of a woman who received the Halachic status of a Zonah because she had relations with a man whom she is prohibited to marry because of an Aseh. (He cites the Rambam, Hilchos Isurei Bi'ah 18:2, who says that a woman can become a Zonah through relations with someone prohibited to her with an Aseh.)

The Aruch la'Ner gives the example of a woman who became a Zonah because she had relations with a second generation Mitzri or Edomi. The Torah (Devarim 23:9) states that a third generation Mitzri or Edomi may marry into the Kehal Hash-m. This implies that there is an Aseh prohibition against marrying a first or second generation Mitzri or Edomi. Therefore, she becomes a Zonah through such a Bi'ah, but it may be that even though, usually, a Zonah is a Lav to a Kohen, nevertheless if she became a Zonah through an Aseh, then she will be prohibited to a Kohen only as an Aseh.

3) The Aruch la'Ner concludes that if we decide the doubt of the Mishneh l'Melech and say that a Zonah created by an Aseh prohibition is herself only a Zonah bearing an Aseh prohibition, this means that we now possess two examples of a Chalalah from Chayavei Aseh: (a) the daughter of a Kohen Gadol from a Be'ulah, (b) the daughter of a Kohen and a Zonah, where the latter became a Zonah through an Aseh prohibition.

4) We have therefore found, bs'd, multiple examples of Aseh that generate Chalalos.

5) There is a totally different Peshat in this Gemara given by one of the Rishonim, according to which the plural form of "Chayavei" is not difficult. This is the explanation of Rabeinu Yerucham (in Toldos Adam v'Chavah Sefer Chavah, Netiv 23, part 4, page 198, top of second column, cited by the Enyclopedia Talmudit vol. 16, end of page 4, DH Af Yisrael, and note 50).

a) He writes that a woman married a Yisrael to whom she was forbidden by an Aseh prohibition. So far no Kohen has been involved, so the Aseh involved can be an Aseh that applies to any Yisrael. However, the woman became classified as a Zonah since a woman who married a man forbidden to her by an Aseh becomes a Zonah, as we saw above (2) in the name of the Rambam (18:2).

b) The aforementioned Zonah then married a Kohen, and it follows that the child is a Chalal. According to Rabeinu Yerucham, this is what the Gemara means when it says that a Chalal results from a Chayavei Aseh. It did not start by the Kohen marrying someone forbidden to the Kohen as an Aseh, but rather by a Yisrael marrying someone forbidden as an Aseh. Since there are multiple categories of the latter, the word "Chayavei" is not problematic.

Kol Tuv,

Dovid Bloom