I walked out on the movie because of that, and it was right in the beginning. Yet for years have noticed no one else has even mentioned it till now. Thank you for saying so!
(Full disclosure: I've never seen the movie, but have read numerous movie reviews of it. Also: In our modern culture, even someone who has never set foot in a cinema will unavoidably "soak up" some of this culture.)
My take is that it was not intended as a covert reference to fellatio. Rather, I think most kids would simply think that it was a "gross" thing to say - and they would say it for pure "shock effect," without any regard for any possible allusion.
But I agree that it is vulgar and does skirt the line.
Regards,