Thailand offers everything under one roof, so to speak. You can get a little bit of everything you get there to some extent in most of its neighbours, but not in such abudance. For example, in Malaysia there's massage places where they do "extras", there's clubs with freelancers, and there are "spas" where you pick a girl from a line-up and fuck her in an adjoining room - but you have to know where to look and it's all a lot more work than Thailand.
Can't you just book a cultural trip to Thailand and bolt some extra days on for mongering? Hell, you can do mongering on said cultural trip. I did a package trip to Thailand as my first ever time there and just fell into the whole red light scene because an older guy in the tour group asked me to join him when he broke off from the group and ended up taking me to be his wingman at bars in Bangkok as well as a go go place in Chiang Mai. That way you come back with millions of photos of you at temples etc and you just leave out what you got up to in the evenings. Do you not have someone you know in the travel industry who could knock up an official looking itinerary for a package that doesn't exist? Like a mate who works for Travel Counsellors or Trailfinders or someone like that? Then if you have a partner or someone that's suspicious of why you're going you can show them this itinerary full of cookery classes, temple visits, train journeys and the like. Put in random off the beaten track names like Lampang, Chiang Rai etc that nobody associates with mongering and you're set. Or just nick an itinerary off a website like Travelbag, Exodus or Explore.
(I will admit, I've not read far enough back on the thread to know why you're so keen to avoid a Thai stamp in your passport so I'm making a few assumptions in this response)