At least some slaves did marry in North Carolina before the Civil War.
For instance, the register of Grace Episcopal Church at Plymouth in Washington county includes a number of marriages, such as "William (servant of C Howcott) & Axy (servant of J B Beasley)" on 18 March 1854. These were separately listed in a section of the register under the heading "Marriages Colored".
Another example is found when Dinah McRae applied in September 1892 for a pension as the widow of Stephen Davis, who had served in the US Army during the Civil War. She stated that they had been married on 15 January 1856 at Tyrrell county by “Clayton (Master)”. Both bride and groom were held in slavery at the time of their marriage. More details appear at: http://www.howcutt.org/catherine.htm