Yevamot 54B

ביומי תליא מילתא. אשת אח ⁠— בבנים תלא רחמנא.

the matter depends upon days, not blood. Although the flow of blood causes her to become impure, she remains impure for a full 7 days regardless of whether she continues to experience menstrual bleeding for the duration of that time period. Similarly, in the case of a brother’s wife, God makes her forbidden status dependent upon children, as, if she has children she remains forbidden to her husband’s brother even after her husband dies.

אלא פריך הכי: מה לנדה ואשת אח שכן אין אוסרן מתירן. תאמר באשת איש שאוסרה מתירה!

Rather, the Talmud refutes the derivation in the following manner: What comparison can be drawn from a menstruating woman and a brother’s wife, which are unique in that what renders them prohibited does not render them permitted? Each becomes permitted due to an external factor, i.e., the passing of 7 days or the death of her childless husband. Can you say that the halakhot of these cases apply to a married woman, whose prohibited status is different in that the one who renders her prohibited to the rest of the world, i.e., her husband, has the capacity to render her permitted through divorce?

אלא אמר רבי יונה, ואיתימא רב הונא בריה דרב יהושע, אמר קרא: ״כי כל אשר יעשה מכל התועבת האלה ונכרתו הנפשות העשת״⁠ — הוקשו כל העריות כולן לנדה: מה נדה בהעראה, אף כל בהעראה.

Rather, the Talmud offers another source for the halakha that the initial stage of sex is considered sex. R' Yona said, and some say it was Rav Huna, son of Rav Yehoshua: The verse states at the end of the chapter on forbidden sex: “For whoever should do any of these abominations, the souls that do them should be cut off” (Leviticus 18:29). All those with whom sex are forbidden are compared to each other in this verse, and therefore also to a menstruating woman, who is mentioned in that chapter. Consequently, just as one is liable to receive punishment for having sex with a menstruating woman from the time of the initial stage of sex, so too, one violates all prohibitions against engaging in forbidden sex from the time of the initial stage of sex.