Dog problem. My male Border Collie has a terrible habit of sniffing women’s crotches, how can I stop him?

He is about 18 months old, and when I walk down the street he has a tendency to make a beeline for women’s crotch areas and sniff away. It’s highly embarrassing. Worst still, when my girlfriend Karen comes round, he not only sniffs, but tries to lick her between the legs while humping her ankle. [...]