If we put aside the pussy joke, I think that the kittens have always been connected to the slayerhood, in one way or another, whether it was about Buffy herself (the big kitten she followed in the desert) about Dawn who was part of Buffy, or about the Potentials who were Buffy's babies.
That just makes the fact that Spike gambled with them and occasionally ate them (potentially anyway--no pun intended) quite funny.