Logline: An impulsive American tourist with an assertive girlfriend back home proposes marriage to a small-town Chinese woman, then blunders into an awkward �menage a trois� when his persistent foreign fianc�e arrives at his doorstep .

This is the Rom-Com approach.

Same story, simple logline revised:  In a modernist "Madama Butterfly", a vacationing American tourist becomes engaged to a small-town Chinese woman, but upon his return home he regrets his hasty romance when his persistent foreign fiancée arrives at his doorstep.  

Bit confusing to be honest. Is the assertive girlfriend the same woman who arrives on his doorstep?

Might be a better adjective than 'assertive' too - like demanding, or high maintenance.
And it sounds like the American tourist acts impulsively by proposing marriage while he's overseas, is that right?

Yes, he, the tourist, proposes marriage while he is in China.  That makes the woman the foreign fiancee.  The girlfriend is back home.  I was trying to be economical.  Thanks for your comments.

The story is a modernist take-off from "Madama Butterfly".

If I'm off base, I will delete this comment... I would not call it Yellow Fever. "Yellow" from what I know is a derogatory term for Asian persons.

AJR

I am glad you noticed.  I put it in quotes, for the time being.  It comes from the Navy Vet  sidekick character as a possible explanation for the protagonists condition.  In Chinese, Huang Bing.  Could go with something like  "Asian Fever".

Maybe:

After a vacationing tourist impulsively proposes marriage to a local, l small-town Chinese woman, he must face the consequences when his long-time American girlfriend surprises him with a visit.

Strong recommend against the title. I wouldn't use "Asian Fever" either. Spike Lee can do it. We cannot.

I think you should build on Dave's recommendation. The "After (x) happens, the protagonist must (y)" structure is always nice and clear because it matches the way the story will progress.