"The word sex made it into the [Civil Rights Act of 1964] at the last minute, almost accidentally. It was inserted only after the drafting and congressional hearings, when the bill went to the House floor. Representative Howard W. Smith, a Virginia Democrat who opposed the bill, introduced an amendment adding sex discrimination, prompting laughter from his colleagues, who mockingly offered other suggested additions. Despite speculation that Mr. Smith meant to weaken support for the bill — he said his concern for women was sincere — his amendment passed, and so did the act. The rights of transgender people never came up."
From "Transgender Fight in North Carolina May Hinge On 1964 Law" (in the NYT).