"Would you like maggots with that?"
The myths fall away like water.
Amongst others, the "Thais have guts of iron" is a demonstrable pile, nay, stream, of spray-painted liquid shit.
Having spent a lifetime ramming fistfuls of peppers and chillies down their gullets hasn't made Thais immune from the bouts of diarrhoea that affects those who are not used to such consumption. In fact, my Thai girlfriend is afflicted on a nearly weekly basis. This doesn't eliminate doggy-style cunnilingus sessions, it only means that they are interrupted by mad dashes to the loo.
The spiciness of Thai food is likely not even the main cause of such bouts. The lack of hygiene at restaurants and food vendors is atrocious in most cases. Hundreds of years after man discovered bacteria and later the importance of refrigeration in reducing risks associated with food preparation, numerous smiling Thai apes who own restaurants still find nothing peculiar about strategically displaying raw meat in the open air, situated under the glaring sun, with flies buzzing freely, presumably to lure in customers who can't resist such an appealing sight. Strangely enough, their marketing ploy seems to work.