The verb be has some "lexical" uses (where it's more like a regular word, compared to the usual "grammatical" be). The most common is to convey a sense of intention:
X: Don't be silly!
Y: I'm not being silly!
(By contrast, the grammatical be never uses do this way: we say "It wasn't a problem", not *"It didn't be a problem.")
Lexical be is usually only found in the forms be and being, and only when grammatical be would use those same forms; so lexical be is a regular verb, except with very restricted use.
In the rare cases that someone wants to use lexical be in a context where be or being doesn't work, they can't just use the corresponding form of grammatical be, so they have to either find another way to say it, or they're stuck using be in a weird way.
In your example, "They are naked" wouldn't work — Judy has already mentioned that they're currently naked, that's not the point that Nick is making — so Nick could either rephrase as something like "They choose to be naked", or he can say "They be naked", as in fact he did. This isn't something you'd see in formal English — it's a somewhat playful or rule-bending use of the language — but it's not unheard-of.
Another example that comes to mind is the movie Bratz:
Yasmin: […] We can be friends with each other and do our own thing, right?
Sasha: Okay. How?
Yasmin: We be ourselves, just like we used to be.
Here as well, it wouldn't make sense to say "We are ourselves"; rather, Yasmin is proposing that they choose to be themselves (in the sense of "be yourself").