Things with a hole in the middle bring headaches

Wisdom: (Those) without scrotums are all very troublesome.

When Dad needs to say something mildly subversive, inappropriate or simply bad, he says it in Cantonese.  This is the first heart-to-heart conversation between father and son.  He switches to Cantonese when he utters the above.  His son Chang Chen on screen (and in real life) has no idea what's saying.  When asked what it means, the Dad's response is so typical (of Chinese?): you'll know when you grew up.  The film is mostly in Mandarin, with some Taiwanese and little Shanghainese thrown in.  There are three instances, which I really don't know why, the dialogues are in Cantonese, the above being the first one, then one appears off-screen between the parents, and one when Dad goes ballistic on his wife in bed, not sexually though.