Halakhah for Keritot 35:35

From the laws of the commandment is that which they, may their memory be blessed, said (Keritot 18a) that there is never a liability for this sacrifice until there is an established prohibition there. For example, that there be two pieces in front of him and one of them is certainly forbidden fat - as behold there is an established prohibition here, and he ate one of them. And so [too,] if he ate a certain forbidden fat, but he is in doubt if there was a whole kazayit or not (Keritot 17a) - this too is an established prohibition. But if there was one [piece] in front of him and it is a doubt if it was forbidden fat or permitted fat, and he ate it - he is exempted from the sacrifice, as there is no established prohibition here; as lest there is no prohibition here at all. And from this reason they said that one who has sexual relations with a woman the divorce of which is in doubt is liable for a sacrifice; as behold there is an established prohibition here, such that we evaluate her according to [that which is] established about her. But one who has sexual relations with a woman the marriage of which is in doubt is exempted from a sacrifice, as there is no established prohibition here; and behold it is like one piece with a doubt if it is forbidden fat or permitted fat, such that he is exempted from a sacrifice (Ketuvot 22b), as we have said. And the rest of its details are elucidated in Keritot (see Mishneh Torah, Laws of Offerings for Unintentional Transgressions 8).