A more thorough explanation:

The Mann Act, also known as the White-Slave Traffic Act of 1910, is a federal law that makes it illegal to transport any woman or girl for the purpose of prostitution, debauchery, or any other immoral purpose.

The Act was passed in 1910 and named after Congressman James R. Mann. It was created to address the problem of human trafficking and prostitution, particularly among immigrant women.

However, the Act has been criticized for its broad language and racist undertones. The phrase "immoral purpose" has been used to prosecute consensual sexual activity, including premarital, extramarital, and interracial relationships.

For example, in the case of Caminetti v. United States (1917), the Supreme Court held that even consensual "illicit fornication" constituted an "immoral purpose." This meant that a man could be held liable for transporting a woman for consensual sex, even if she consented.

The Act has been amended several times, including in 1978 to address child pornography and in 1986 to clarify that it should not be used to prosecute consensual sexual activity.