Yevamot 76: Those Who Can't Marry In

Two mishnayot: Several parties are prohibited from marrying those who are born Jewish, but can still marry converts to Judaism or freed female slaves. Beginning with a male with damaged genitalia and shifting to the Givonim, who have a rabbinic decree against them, or at least those who are capable of having children. Also, converts from Ammon and Moav are forever prohibited from marrying women who are born Jewish, while their female counterparts are allowed to marry men who born Jewish. Edomite and Egyptian converts are prohibited for 3 generations only. But the practical halakhah is likely different. Plus, the attention paid to David (who is not yet king) and his lineage - on both sides.