Are you saying that's the only place on your body you have pimples? Are you sure that's what they are?
Where are they, exactly? Are they on your penis, in your pubic hair? If they are in your pubic hair, I wonder if you have folliculitis, which is infected hair follicles, caused by a bacterial infection. You may need an antibiotic, either a cream or a pill.
If it's on your penis, are you sure they are pimples? There is something called Fordyce spots, which are just oil glands, and they are totally normal, and not pimples.