"Why, oh why, do an entire episode about Mitchell trying to overcome his PDA issue and then not have him kiss his partner?"
They did kiss, but in the end it was actually downplayed. It was at the end of the scene when the family confronted their issues with PDAs, and Cam & Mitchell kissed in the background. All in all, I thought they handled it pretty well. We'll see if their PDAs become as common as the other adults...
"It does me no injury for my neighbour to say there are 20 gods or no god. It neither picks my pocket, nor breaks my leg."
-- Thomas Jefferson