I had not fully appreciated that the criteria laid down in the United States White-Slave Traffic Act of 1910, aka the Mann Act, were as stringent as would appear to be the case. Here’s the Wikipedia entry on the subject, while this source purports to give the full text of the legislation concerned.
I did read somewhere, but cannot recall the source, that an amended version of the Mann Act is still in force. I’m curious to know what has changed. For example Caminetti v. United States (1917), as cited by Wikipedia, determined that consensual extramarital sex should come within the boundaries of immorality, so let’s say that I’m on holiday in California and I meet a married woman. She’s separated from her husband so in my book it’s morally OK, not that it matters for the purposes of this question. We agree to visit Las Vegas for a few days of total fun.
Would our trip be illegal and, if not, when did it become legal?
Note that the the offense, as applicable to adult transportees, is limited to prositution or sexual activity for which any person can be charged with an offense. If all you are talking about is extramarital sex, it’d have to be illegal in one of the states involved. Neither Nevada nor California appears to have a criminal statute prohibiting adultery (unless it’s incestuous). Mann Act - Immoral, Criminal, Sexual, and Prostitution - JRank Articles
There’s also a question about whether *Lawrence v. Texas * invalidates adultery laws. I’ve argued elsewhere that I don’t think it does.