Shevuot 18B

ואנן במקדש תנן! אלא משום דלא דמי; ארוכה דהכא ⁠– קצרה דהתם, וארוכה דהתם ⁠– קצרה דהכא.

And we learned this difference in the Mishnah specifically with regard to ritual impurity in the Temple. If such a distinction were also in effect with regard to a menstruating woman, the Mishnah would mention it. Rather, one can explain: No parallel distinction is made, because the two cases are not similar in their details. The long way here, with regard to a menstruating woman, namely, that the man must wait, is like the shortest way there, with regard to impurity in the Temple, namely, that the impure person must leave the Temple by way of the most direct route. And the long way there, with regard to the Temple, is like the shortest way here, with regard to a menstruating woman.

מתקיף לה רב הונא בריה דרב נתן: מי אמר אביי אנוס הוא ⁠– אלמא בשלא סמוך לוסתה קאמרינן?! והא אביי דאמר חייב שתים ⁠– אלמא בסמוך לוסתה עסקינן!

Rav Huna, son of Rav Natan, objects to what Abaye said: Did Abaye really say with regard to the Mishnah that if the man withdraws with a flaccid penis he is exempt because he is considered a victim of circumstances beyond his control? Apparently, then, we are speaking of a man who had sex with a woman not near the expected date of her menstruation, and therefore the situation is considered beyond his control. But is it not Abaye who says that he is liable to bring two sin-offerings for this transgression, one for his initial entry and one for his withdrawal? Apparently, we are dealing with a man who had sex with a woman near the expected date of her menstruation, so he is considered an unwitting transgressor, who is liable to bring a sin-offering, and is not the victim of circumstances beyond his control. Consequently, Abaye’s two statements contradict each other.