The dreamy look in Jon's eyes and general air of being out of it tells me Jon doesn't quite realize what he's saying. He thinks he's relishing the time spent alone with Liz, but his line of reasoning has nothing to do with the company at the movies. What Jon most enjoyed was time away from Garfield. It's actually fine, good and probably healthy, for Jon to realize this. It's a poor thing to subject Liz to, however, and the ideal result in a developing human being would be to make sure future dates are not just to get away from Garfield.
Part of how Garfield asserts his authority and deeply integrates himself into the Jon's life is to force the man to ingest parts of his body. The vagary of Garfield's angry retort is part of the joke. How can Garfield make good on such a threat? By either sabotaging Jon's dates so he has to stay home and eat the tainted house supply of popcorn, or by violating the entire concession stand at the theater. Perhaps the theater the reader frequents. The heart of a gross-out joke is to ask the audience to imagine themselves with a greasy, salty, crunchy mouthful of fluffy popcorn sprinkled with white flecks of cat dander, and matted with buttery hunks of golden orange fur which stick to their shiny lips and slick fingers.
Panel Three Art Examination: Perhaps it is Garfield's massive right forearm blocking the view, but it appears the limb has become disconnected from our hero's body.