Make the simple syrup. In a small pot on medium heat, bring the sugar, water and rosemary mixture to a simmer. Simmer for 30 mins or so until it gets thicker and syrupy.
Juice the grapefruits until you get 2 1/2 cups. Strain the juice. You can add a little pulp in if you want to, but its best to strain at least most of it. Add the rum and simple syrup. Stir well. Pour into a container and place in the freezer. Freeze until solid.
When the sorbet is ready, scoop 3 to 4 tbsps in each glass and top with sparkling wine or champagne.