The keyboard is not the only way that MacBooks dissipate heat. I don't think that is its secondary function either. There is vent at the back-bottom, near the black strip.
When the MacBook is open, the heat comes out of that vent and the opening between the keyboard and the screen.
Apple even has a support article that gives instructions on how to use MacBooks in this way. They call it "clamshell mode" - http://support.apple.com/kb/ht3131
My guess about the increased temperature is that the GPU is working harder to drive a larger monitor, but that's normal.