There's a google answers thread that has a digression onto how testicaels hang and these two theories:
I could be wrong, but I think the original questioner was referring not to the testicles but to the penis. In most men the flaccid penis points slightly to one side rather than straight ahead. The direction it points in correlates with what side the heart is on -- which is on the left in most people. Thus most men hang slightly to the left. One sign of dextrocardia (having ones heart on the right side of the chest instead of the left) is a penis that hangs to the right.
And
in high school, one of my friends and i asked a number of our guy friends which way they dress, just for fun (we had to explain it to a surprisingly high number of them). anyway, it did seem to depend on handedness as most right handed guys we asked said they dress left while the left handers said they dress right.