Rashi: They are heated with semen under Elah and Alon (oak trees).

Radak: The Nevi'im depict Yisrael idolaters to a woman who is Mezanah under her husband, especially in Sefer Yechezkel, and the beginning of Hoshe'a. So it is here. Necham is Nif'al; the root is Yecham, like "b'Chol Yecham ha'Tzon" (Bereishis 31:10). Intimacy of a male and female is called heating, like "Yechemasni Imi" (Tehilim 51:7), "Yechemu ha'Tzon", "Leyachmenah ba'Maklos" (Bereishis 31:39, 41). Elim is a Klal for all trees; after this, it specifies "Tachas Kol Etz Ra'anan"; refer to 57:5:3:2.

Malbim: This explains "Yildei Pesha" (4). The mother said that gods under the fresh trees had Bi'ah with them, and they gave birth from them. They said that the gods desire them - "Asher Yavo'u Bnei ha'Elohim El Bnos ha'Adam v'Yaldu Lahem" (Bereishis 6:4).

Nidah 13a: One who emits semen unnecessarily is like a murderer. We read "Shochatei ha'Yeladim" like Sochetei. (They cause their semen to exude.)

Rashi: It is to offer them to idolatry. Radak - so Yechezkel said, "uv'Shachatam Es Beneihem l'Giluleihem" (23:29).

Malbim: The slaughter is attributed to the gods. The women said that the gods who fathered the children commanded to slaughter them under the cliffs for a Korban; via this, the children will be sanctified. This is known from the ancient [idolatrous] Avodah of Tzavah and their follies.

Nidah 13a: One who emits semen unnecessarily ¹ is like an idolater. It says here "Tachas Kol Etz Ra'anan", the same expression used regarding idolatry.

Radak: Their practice was to serve idolatry under fresh trees, that their branches and leaves are moist.

Rashi: They are clefts in rocks. The same applies to "El Se'if Sela" (Shoftim 15:1).

Radak: They are rocks under the Se'ifim, i.e. cliffs. The same applies to "bi'S'if Sela Eitem" (Shoftim 15:8); it is like Shen Sela (a peak or cliff), for it is to the rock like a Sa'if (branch) to a tree (both jut out).