should make clear that I don't want to imply that all Thai prostitutes are trouble or all "ordinary" Thai girls are true-hearted.
There are gold diggers everywhere. You have to ask why a girl is attracted to a relationship with you, a foreigner, especially if she is quick to it. It is my experience that most ordinary Thai girls are not after my money (and don't take any), but I am picky and patient. Your mileage may vary. A sizeable percentage of non-prostitutes are attracted largely to the farang's money and material wellbeing, and/or some Hollywood-based illusion they may have of life in a farang's country. They may simply look to a farang to take care of them, everything, whereby they'd "have it made in life". Matrimonial prostitution (though they don't see it that way).
Notably, I usually don't make introductions and wouldn't be seen with many of the farangs who have expressed wanting a good Thai girlfriend (often in a fleeting moment of frustration over the behavior of a prostitute girlfriend they've become emotionally attached to), because I don't think those farangs really understand what they're getting into when it comes to good Thai ladies, would be unlikely to develop or enjoy such a relationship due to inability or unwillingness to adapt culturally (as well as the Thai lady's inability or unwillingness in many cases, too), and/or would be unfaithful to the good Thai girl which in many cases is damaging to her reputation and sometimes her life (e.g., offspring abandoned by the farang). In any introduction, I take no responsibility for anything. Use at your own risk.
Nevertheless, I sometimes do bring farangs into my realm to hang out with, sometimes deeply. Then, whatever happens, happens...
Before a farang even considers a "good" Thai girlfriend, he should consider the ramifications. He will have to give up the bargirl adventures in his life. (Some women put up with their boyfriend engaging in shorttime liassons behind their back as long as they come home on time every night, but it's not true that this is generally accepted in higher class Thai society. MOST Thai ladies do NOT tolerate this.) If he changes his heart and mind about his current girlfriend, he won't be able to dispose of his good Thai girlfriend cleanly with a 1000 baht note. He won't be able to get into her pants on the first night, and probably not the first few nights. Whenever he gets that far, then when he does get to her panties, she will usually be shy (depending upon the friendship trust and art), and it won't be the same kind of experience as with a prostitute, for better or for worse.
In my opinion, quality beats quantity. Of course, just my opinion, different strokes for different folks.