Our Gemara discusses the question of whether it is appropriate to marry a duma or the daughter of a duma. A duma is an Aramaic word that – in our context – refers to a woman about whom there are rumors that she is unfaithful to her husband. Shmuel permits marrying such a woman, but not her daughter, who may be the product of a forbidden relationship. Rabbi Yohanan permits marrying the daughter, about whom there are no rumors, and forbids marrying the mother, who has developed an unenviable reputation.
Rashi and others make the point that neither Shmuel nor Rabbi Yohanan actually forbid marriage to these women, but rather offer strong recommendations against marrying one or the other. It appears that the point of argument is whether the greater concern is with the daughter, given her uncertain status that will never be resolved, or with the mother, who gives every reason to be concerned lest she commit adultery when she is married.
Our Gemara comes to a clear conclusion that marrying the daughter is permitted, since – as Rav Tahlifa bar Ma’arva taught Rabbi Abahu – even if we are certain that a woman committed adultery, we do not cast aspersions on her children, since we rule rov be’ilot ahar ha-ba’al – that the majority of her sexual encounters were with her husband, so the children are most likely his.
The Rambam (Hilkhot Isurei Bi’ah 15:20) accepts the Gemara’s ruling and concludes that a person can marry the daughter with no compunctions (unless she is perutzah be-yoter – unusually promiscuous). The Me’iri, however, deems it appropriate for a person who wants to avoid unpleasant situations to avoid both the mother and the daughter since we have reason to suspect that the daughter will follow in her mother’s footsteps.