Well, Dicho, I decided to see if I could find something about that. Apparently it is always John 3:16 which they quote. There's an interesting comment on it about half way down this page: from a Catholic perspective as near as I can tell. I didn't explore it a lot.

And then, there was this comment from Misc. Media:

(My fantasy: To hold up signs displaying the verse numbers for the passages about Onan spilling his seed, or David spying on the bathing Bathsheeba, or a sequence of the verses that turn out to be "And Judas went into the potter's field and hanged himself," "Go thou and do likewise," and "Whatsoever ye do, do so quickly.")