What's funny about "Looking gay people up in the directory"?

Johnny: "I have a cousin that's gay, by the way."

Tim: "Most people do."

Frankie (shouts): "Ready!"

Johnny: "He's a really great guy."

Tim: "I'm sure."

Johnny: "He just found out he was gay a couple of months ago."

Tim: "I'll look him up in the directory. Under the new listings."

This dialog was while Johnny was waiting on Frankie to ready herself hidden in the bathroom. Tim and his boyfriend were sounding out Johnny for her, and it was uncomfortable, especially once Johnny realizes that Tim and his boyfriend (behind the TV) were a couple. Johnny reacts like most hetero's have in that uncomfortable situation, "I have a friend" or "My neighbor knows someone who is..".

Johnny goes on to say his cousin "just found out he was gay", like shazzam! he didn't know it before, but wow, now he does and so he's just 'come out'.

Continuing in the smart, smart aleck way he had been, Tim (Nathan Lane) makes another joke about it, such as "I'll look him up, you know, in the (newly gay) listings", like there is some directory of all newly gay people.