Different opinions whether having had a forbidden relationship forbids one from marrying a Kohen. Does it depend on whether the situation would have caused a problem for the offspring or not? The gemara then proceeds to understand the final opinion brought in the mishna that questionable people can’t marry each other nor can they marry mamzerim. Rav and Shmuel have a difference opinion about who to hold like and it contradicts their opinions elsewhere. Various explanations are brought to resolve the contradiction.