Ah yes, the Thai boys can be exceptional! If you can find it, the next time go for a B-course massage:
You lay on your stomach, often on an air mattress where soap suds are applied. The boy lays on his stomach on top of you. The massage consisits of his using his whole body gliding over your back using the suds as a lubricant. Then, without you moving an inch, he glides under your body and does much the same from your underside. Sexy as hell! Near the end of this process penetration occurs and fluids exchanged.
A couple of variations on this include the boy washing you down before and after (shower together) and/or a table massage afterwards. There are few places where you can get a B-course massage and fewer still that perform it properly. Expect the cost to be as much as double a regular massage.