Rory, my beloved Great Dane is constantly pulling the leash. Please help.?

Rory, my beloved 5 year old Great Dane has become intolerable when I take him for walks. He is constantly sniffing women's crotches and pulling me into people's yards when they are having BBQ's. When he sniffs something (such as a pile of doggie doo, or a woman's crotch) he plants his feet and digs in and no amount of pulling will remove him from the task at hand. It is so embarrasing. He needs excercise, but he often pulls me across the highway to investigate a dog on the other side of the road! Please offer advice, because I dread going for walks now.