In this strip, Jon tries to reconcile his former glumness about women with his recent successful relationship with Liz. In the first panel, Jon repeats a customary lament about the supposed inability of the sexes to relate to each other. In the second panel, something clicks, pops, or lights-up inside Jon, and he realizes the accepted truism doesn't ring true. There are platitudes that are not cosmic truth, but trite ways to justify whatever bad mood you're in, and I think Jon recognizes the tinge of shrug-shouldered misogyny in "women are a mystery".
But Jon doesn't discard the idea entirely. He appends it with affection and admiration that closer approximates his experience with Liz. The new, weighted shift in meaning still holds, even if the punchline is motivated by a general affection for women and not Liz specifically, though that makes it sweeter. Women may be a mystery, after all, but Jon cannot pretend to be irritated or frightened of the idea. His method of expressing this is silly, but the sentiment is strong and positive. Little wonder, then, that Garfield would feel the need to belittle it, and therefore Jon and Liz's relationship, and by proxy all those with romantic interest in women. We don't need your hate speech, kitty cat!