Actually, Americans have only been intermarrying in large numbers in the last couple of decades. Previously, even Jewish communists and gangsters married Jews. Jews lived in Jewish neighborhoods and had mostly Jewish classmates in public schools as they were neighborhood-based and Catholic parents generally sent their kids to parochial schools. People generally know what their parents and grandparents were so once they become observant they know if they are halachically Jewish and go to a rav.
Anyway, there is a halachic principle משפחה שנמעה נטמעה . Who knows what happened in someone’s family a few centuries ago? It could be that we are all mamzerim as during the time of the First Temple adultery was rampant (see Ulla’s response to Rav Yehuda’s brother Rav Yitzchak on Kiddushin 71b),
Menu