Kiddushin 73 - Whom Can a Convert Marry?
Rava lectured in Mechuza: "A convert is permitted to marry a daughter of a Kohen." The people of Mechuza loaded him with fine silks. He then taught them: "A convert is permitted to marry a woman who is a mamzer." They said to him: "You have ruined the effect of the first statement you made."
Shetuki (he whose unmarried mother silences him when asked about the identity of his father) should not marry an Israelite of unblemished lineage. Even though he is Kosher, since most of the men who might have been father are Kosher, this is an additional measure to protect genealogical purity.
Art: Edwin Lord Weeks - The Silk Merchants