Ketubot, Chapter One, Mishnah Eight
This mishnah discusses a woman who is suspected of having had relations with a man prohibited from marrying an Israelite and she claims that while she did have relations with her, he was not the type of man prohibited from marrying an Israelite. Again there is debate over whether or not she is believed.
They saw her talking with someone in the marketplace, and they said to her, What sort of a man is he? [And she answered, He is] the so-and-so and he is a priest
Rabban Gamaliel and Rabbi Eliezer say: she is believed,
And Rabbi Joshua says: we do not live by her mouth, rather she is in the presumption of having had relations with a natin or a mamzer, until she brings proof for her statement.
In the Talmud there is debate over what the woman was seen doing. According to some, she was seen having intercourse with an unknown man, and that talking is a euphemism for sex. Others say that she was merely talking with him, but there was suspicion that they had had sex. It is important to remember that the situation is that she is a single woman and there is no issue of adultery. However, if she had had relations with a man who was forbidden from marrying an Israelite, such as a mamzer or a natin, she would subsequently be prohibited from marrying a priest. When asked who this man was she provides his name and says that he is a priest. Note that it is not essential that he is a priest; it is sufficient that he is a man who is not prohibited from marrying an Israelite.
Rabban Gamaliel and Rabbi Eliezer again rule that the woman is believed and may subsequently marry a priest. Since she has a presumption of being fit to marry a priest, it requires solid evidence to remove her from this presumption.
Rabbi Joshua holds that since she was secluded with him, she loses her presumption of being fit to marry a priest. She must bring proof that this person was not prohibited from marrying an Israelite and until then she may not marry a priest.