Depends on the guy and the place to a certain extent.

My recent experiences at Banana were quite pleasant. They have regular massage downstairs and erotic massage upstairs. Cost is a little bit on the high side and there is a minimum tip set forth. Upstairs the guy will shower with you before and after. Some are better at massage than others if you want an actual massage with your fun. Both guys I met upstairs on my most recent trip were very sweet. Both were pretty skinny, boyish types. One was surprisingly large. Second guy was pretty much willing to do anything. I also had a regular massage there downstairs one evening. I was pretty stiff after my long flights so it was nice to have a real massage. The guy was fairly young and handsome but no hanky panky. Very heavy pressure which in retrospect was a little too hard.

Pretty much every place I have been to in Bangkok has been a good experience. If you want essentially a sex massage then go to one of those places and don't go to a regular massage shop expecting more.