You are right of course - people have different perceptions; for me, diving is definitely the thing that comes to my mind from Thailand, even if I have never done any scuba diving myself. The next things are beaches, food and the tsunami.

But regarding what Google typeahead suggests to you, it depends on your location and perhaps your own search history. For me the options with "Thailand se" are september, seasons, sevärdheter ("sights" in Swedish). So if someone sees proposals with "Thailand sex", it could be because he or she has been doing previous searches for Thai sex tourism.