Are you using the 2D exchange export or the DXF/DWG export? Or something else? You may need to adjust one of your settings files.
An old trick for exporting DXF/DWG was to export your drawing as a CGM file, start a new (blank) NX part file, import the CGM, and finally export the DXF/DWG from this file which only contains curves in the modeling application.