I'm very strongly right handed, and incapable of doing any task requiring dexterity with the left hand. However, I can only jerk off with the left hand. I don't mean that I just prefer the left; if I use the right hand, I cannot cum.
Most of my fuckbuddies cum before I do, so after they cum, we usually cuddle while I jerk myself off, and blow a load all over them. As Murphy's law would have it, much of the time, they tend to cuddle up to my left side, so I can't reach myself with the hand that works.