Yevamot 70A

אבל דיימא מניה, אף על גב דדיימא מעלמא ⁠— בתריה דידיה שדינן ליה.

However, if she is rumored to have had sex with him, even if she is also rumored to have had sex with others, we cast the child after him.

אמר רבא: מנא אמינא לה ⁠— דקתני: ילדה ⁠— תאכל. היכי דמי? אילימא דדיימא מניה ולא דיימא מעלמא ⁠— צריכא למימר דתיכול?! אלא לאו ⁠— דדיימא נמי מעלמא,

Rava said: From where do I say that? My source is the Mishnah that teaches that if a priest engaged in extramarital sex with an Israelite woman and she gave birth, she may partake of teruma due to her child. What are the circumstances? If we say that she is rumored to have had sex with him and she is not rumored to have had sex with others, need it be said that she may partake of teruma? It can easily be assumed that the priest is the father. Rather, is it not a case where she is rumored to have had sex with others as well?

ומה התם, דלהאי איסורא ולהאי איסורא ⁠— בתריה דידיה שדינן ליה, הכא, דלהאי איסורא ולהאי היתירא ⁠— לא כל שכן?!

And if there, where for her to have sex with this priest is in violation of a prohibition and to have sex with that non-priest is in violation of a prohibition of the same degree, and she is rumored to have had sex with both, nevertheless, we cast the child after the priest, then here, where for her to have sex with that man who is not her betrothed is in violation of a Torah prohibition and to have sex with this man, her betrothed, is permitted by Torah law, is it not all the more so that he should be considered the father?