What's the meaning behind this joke in George Carlin's 1977 HBO special?

Question

In his 1977 HBO stand-up special George Carlin at USC, George Carlin tells the following joke about cats kneading:

And they jump on your chest, they put their ass in your face and go...

[mimics a cat kneading]

[in an annoyed voice] What is that, get him off me, will ‘ya? I don’t even know what that is and I don’t like it. I think it has to do with ecstasy or something.

[in a woman's voice] He misses his mommy.

[in his normal voice] Yeah, bullsh*t, you always say that. You said that about the mailman.

[audience moans]

What is the meaning behind You said that about the mailman? Why does the audience moan?

Answer 1

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.