This article is discussed widely on the Thai forums around Thailand and most of the people do not even believe the couples were raped at all.
I won’t publish the text here but you can read it your self here
http://www.dailymail.co.uk/news/worldnews/article-1231589/British-couple-subjected-vile-Clockwork-Orange-style-sex-attack-Thailand-holiday.html
First of all Kho Chang is an Island there is nowhere to run or hide like on the mainland.
Secondly they saw the people that did it to them and they know what bar they hang around in.
But sure strange things happen when you are on holiday, maybe it was a meeting of swinging couples that went wrong, or they were actually was doped and raped.
But if this people actually live on the Island of Kho Chang more people would have noticed it and the hospital on the island would know about it. I mean how many people can you date rape per week before the police actually do something?
Even if this thing really happens I think Thailand is a country when rape of white females and males is pretty rare if you compare to other countries. ]
Another problem is the Thai police really do not know how to deal with rape even when it comes to Thai women being raped by Thai men. The Thai law just two years ago made it a crime for the husband to rape the wife.