The male reproductive equipment is cooler than the rest of the body. That's why it is so sweaty there. Humidity accumulates and growies get a hold. Growies itch, and a man needs to scratch. But that has nothing to do with the image you posted.
Im not talking about the scratching that takes place indiscreetly, behind a tree or something, im talking about when you happen to walk up to a guy and start a conversation . Some of them automatically yank at there yanker, I find it rather interesting.
A groin is a mason's term, the line defined by the meeting of two vaults. The same line is found where the leg meets the torso, and many other examples.
A few reasons: Itching. If the guy shaves down there, it's excruciating. A misaligned jock. If a sportsman grabs himself there, that's almost certainly why. Trap a ball outside that thing and yo' in deep kimchee. Hiding a badly timed erection. Us guys can be embarassingly semaphoric. You girls can hide it easier, but a schlong is much bigger than a nipple, it shows. To hit the high notes, that's why MJ did it.
Don Barzini 