In US v. Bitty (decided Feb. 24, 1908), the US Supreme Court, in a decision written by then Associate Justice John Marshall Harlan:

“There can be no doubt as to what class was aimed at by the clause forbidding the importation of alien women for purposes of ‘prostitution.’ It refers to women who, for hire or without hire, offer their bodies to indiscriminate intercourse with men. The lives and example of such persons are in hostility to ‘the idea of the family as consisting in and springing from the union for life of one man and one woman in the holy estate of matrimony; the sure foundation of all that is stable and noble in our civilization; the best guaranty of that reverent morality which is the source of all beneficent progress in social and political improvement.'”

Feb. 24, 1908 - U.S. v. Bitty