Geographic routing has been proven to provide efficient and scalable unicast routing in resource-constrained sensor networks. However, its applications in multicast routing remain largely unexplored. In this paper, we propose the DCGM (Destination Clustering Geographic Multicast) routing protocol for wireless sensor networks. By clustering destinations that can share the same next hop, we iteratively select the next hop as the neighbor with the maximum number of destinations. We prove that the complexity of DCGM is O(nk), where n is the number of neighbors of the current node and k is the number of destinations associated with the current node. Simulations show that DCGM achieves better performance than existing geographic multicast algorithms in terms of path length, but with much lower computation complexity.