How odd! I've always seen Lureen as sassy or feisty and quite able to complain and bitch. Jack has some of the same aspects - I had assumed their marriage might be not based on physical passion after the first months but contain a certain amount of conflict or mutual irritation. I never thought of it as being milk and water in that sense.
KitKat, thanks for the translations/explanations provided. I really think any examination of this proverb should be linked to the original Chinese context, which is surely what Lee had in mind. Coming together effortlessly, complete union, lack of friction. Hmmm.
My idea/assumption is that when Lee told JG and AH that they "come together like milk and water," he's talking about the scene in the back seat of the car. It's an effortless, gravity-propelled, 32 feet per second per second ("Do you think I'm too fast?") descent into physical satisfaction.
And afterwards.... They're both young and attractive... they think, "why not?" and they get married. The milk and water metaphor does not apply to their marriage, I don't think, especially as Jake gets to know the family...
Finally, the image changes, "As far as our marriage goes, we could do it over the phone."
But that first meeting.... milk and water.