Dan,
First, the stand of "Who are we to comment about what constitutes rape in the South Asian cultural context" would have merit if the South Asian/Bangladeshi norm was that *** on false promise would actually be considered rape. This, however, is patently untrue. It was not the intent of the legislature while framing the 'consent' laws that false promise would be interpreted within the legal framework for rape in the modern world devoid of context. Rape charges like this are widely considered a joke both by society and legal academic circles.
It is definitely true that reneging on a marriage promise obviously has drastically different connotations in BD and in the western world and a societal argument can be made for punishing false promises of marriage (though I wouldn't subscribe to it). However, that has to constitute a completely different legal framework and trying to hijack it using a super technical interpretation of rape laws is something which is thoroughly ridiculous. It is not a societal differences issue as you have framed it but an insufficiency in South Asian law regarding the interpretation of consent which has led to vexatious litigation which should be rectified within the next few years, at least in the Indian context.
Additionally, you obviously agree that there are certain types of laws in countries which should be universally looked down upon. A 5 year jail sentence/caning for possession of marijuana or say, imprisonment for homosexuality. Rigorous imprisonment, the deprivation of enjoyment of life and liberty for a charge such as *** on false promise of marriage is something you're comfortable with, regardless of where in the world it might be?
Basically, the false promise *** issue, particularly with regard to rape conviction is neither jurisprudentially nor societally nor logically nor morally accurate. It exists due to a technicality in interpretation of law.
It's the bastard child of blackmail litigation and context-less interpretation of statute and having the issue of rape in the same hemisphere does a lot of damage to rape laws which are an extremely important legal protection.