You're right but up until 1920 US citizenship was immediately conferred upon a foreign woman by marriage to a US citizen -- so if the father was a US citizen then the mother became one as well instantaneously.
It was called derivative Marriage by the ‘Act Of February 10, 1855’.
http://www.archives.gov/publications/prologue/1998/summer/women-and-naturalization-1.html
The act of February 10, 1855, was designed to benefit immigrant women. Under that act,
“[a]ny woman who is now or may hereafter be married to a citizen of the United States, and who might herself be lawfully naturalized, shall be deemed a citizen.”
Thus alien women generally became U.S. citizens by marriage to a U.S. citizen or through an alien husband’s naturalization.
Act of February 10, 1855
1. Women take on the citizenship of her husband through derivative citizenship. She proved her citizenship with her marriage certificate and her husband’s certificate of naturalization.
Total residency in US: 5 years; 1 year in state; 2 years between declaration & petition