Комментарий к Берешит 19:36

ותהרין וגו AND THEY BECAME PREGNANT, ETC. — Even though a woman does not become pregnant from the first intercourse, these [women] mastered themselves and took out their hymens; and they became pregnant from the first intercourse (Genesis Rabbah 51:9).

ותהרין, they each became pregnant as a result of one sexual union with their aging father. As soon as they became aware of this, they desisted from further such contact with their father.

These did it with themselves... שלטו means “had relations,” since לרבעה is translated by Onkelos as למשלט. In other words, they “had relations with themselves” and removed their virginity [beforehand. This enabled them to conceive]. This is the meaning of והוציאו ערותן לחוץ. Some versions of Rashi say והוציאו עדותן (“they removed their evidence”), which means the same. It refers to their virginity, which are the evidence they are virgins, as it is written (Devarim 22:17) אלה בתולי בתי, which Targum Yonasan explains as אלה עדות בתי.