פירוש על קידושין 142:6

R. Yehudah does not seem to want to marry off his son because he cannot determine the lineage of the women around him. Ulla begins to point out that we too do not know our lineage. Perhaps we are descended from the Babylonians who raped Israelite women during the time when the First Temple was destroyed. R. Yehuda could respond that he holds that the offspring of a non-Jew and a Jewish woman is fit. Therefore, maybe we are pure.

In this colorful exchange, R. Abbahu ends up interpreting the verse about those taken captive as referring to people who act licentiously, committing adultery. Perhaps, Ulla says, we are descended from such people whose children are mamzerim.
In short, Ulla questions how R. Yehudah can be so sure about his lineage.