Marc, I agree with you fully that it isn't really very decent for a farang guy to let a girl from a third world country (to use your words) pay for both of you if you go out with her. Even if it is her wish to do so. That is a no no for me.
If it is for small things, like water, fruit or a motorbike taxi and she insists, I could accept, but even then I will make sure she will be compensated financially in a different way. This is also how my gf treated me and v.v.
However, I must say that even on business trips where the company involved would eventually pay for the bill, the lady who took me out, gave me cash prior to go into a restaurant, so a man could pay the bill and not a lady. This happened to me a couple of times. For example in Russia, Singapore and even in Germany for that matter.