פירוש על בבא בתרא 176:18

But here there is no possibility of repentance. And this is said specifically regrading the sexual sins that lead to no mamzer, but one who has intercourse with a married woman and gives birth from her, or from one of the other forbidden close relations, this is "a crooked thing cannot be made straight" (Eccl. 1:15) as it is said (Yevamot 21a) regarding teshuvah 'and behold Rabbi Chayim says that even though what is crooked cannot be made straight it is possible to do teshuvah as commanded by God, Who has commanded him to do teshuvah and sincerely repent, but a robber, whose teshuvah is dependent on returning what was stolen, as it is written "and he returns what he stole, etc." (Lev. 5:23), how can one who stole from the public know exactly from whom he has stolen to return [the property]? And even though it is said 'let him pay with [the stolen goods] the needs of the community' (Baba Kamma 94b) in any instance it is not complete teshuvah.