You have to look at different versions/transcripts to get the full meaning of that passage. While I haven't seen that particular one, I have heard this skit in a different format:
Or sometimes if he’s on the bed with you he’ll climb onto your chest and stick his ass right in your face: “Hey, here’s my ass! Check my ass, Daddy! Get a nice, clean look at my ass!” And then while he’s showing you his ass, he starts that kneading thing with his paws; like he’s playin’ the piano. God, I hate that. “Get him offa me! Jesus, I hate that! I don’t even know what it is, and I hate that. It’s as if he got hold of some bad drugs. What is that?” “It’s an instinctive nursing behavior, honey. He misses his mommy.” “You always say that. You said that about the mailman.”
The implication is that he came home and found the mailman doing the same behavior with his wife, and she dismissed it as instinctive and missing his mommy, i.e. she was fooling around with the mailman and got caught, and this is another reference to that.