Yevamot 95B

אלא אמר רבא: אשת איש. וכן כי אתא רבין, אמר רבי יוחנן: אשת איש. ומאי קרי לה ״איסור קל״⁠ — שאין האוסרה אוסרה כל ימיו. תניא נמי הכי: אבא חנן אמר משום רבי אלעזר: אשת איש.

Rather, Rava said that the light prohibition is actually that of a married woman. And similarly, when Ravin came from Eretz Yisrael, he said that R' Yoḥanan said that the baraita is referring to a married woman. And for what reason does the tanna call this a light prohibition? The reason is that it differs from and is more lenient than other prohibitions in that her husband, who renders her forbidden, does not render her forbidden for his whole lifetime, as he can negate the prohibition by giving her a bill of divorce. This is also taught in a baraita: Abba Ḥanan said in the name of R' Elazar: The baraita is referring to a married woman.

ומה במקום הבא על איסור קל, שאין האוסרה אוסרה כל ימיו ⁠— נאסר האוסר, הבא על איסור חמור, שהאוסרה אוסרה כל ימיו ⁠— אינו דין שנאסר האוסרה?

The Talmud explains that according to this opinion the a fortiori inference should be understood as follows: And if in a case where he has sex with a married woman, which is a light prohibition, due to the fact that he who renders her forbidden does not render her forbidden for his whole lifetime, and yet the one who renders her forbidden is forbidden, as the husband of a woman who committed adultery is permanently barred from having sex with her, then the following conclusion is correct: One who has sex subject to a severe transgression, e.g., a wife’s sister, which is severe because the one who renders his wife forbidden renders her forbidden for her whole lifetime, since as long as his wife remains alive he is forbidden to her sister, is it not right that the one who renders her forbidden should be forbidden?