Marital rape became a crime in all 50 states when North Carolina became the last state to pass such a law. The first spousal rape law was passed in North Dakota in 1975.
The definition of rape was most commonly defined as “sex with a woman, not his wife, without her consent.” The “logic” goes back to 17th century England when a Chief Justice gave the opinion that once married, a woman “hath given herself in kind to her husband which she cannot retract”, in effect ruling that once married, a woman does not have the right to refuse sex with her husband. This remained unchallenged for over 300 years until the feminist movement of the 1970s when women argued this failed to provide equal protection under the law for all women.
For a much longer writing of the horrors of these marital rape laws, see my blog at: