While this is one possible interpretation of the situation at large, I believe the humor is really found elsewhere. You see, many individuals use their laptops to view explicit pornographic materials. To access said media, one must use the various keys and pads on said laptop. Since most people would choose their dominant hand to rub one out, and no one wants cock juice anywhere near their keyboard, they choose to use their non-dominant (often left) hand to navigate web pages. Very very humorous really!