This is a no brainer. YOU HAVE TO TELL HIM.
You messed up. The reason people are telling you not to tell him, is because they know you messed up; and as long as you keep it a secret, you won't ever get any repercussions from your boyfriend.
Yet. You feel you know you should. Do you plan on being with this person for the rest of your life? This is going to be something you are going to hold inside a closet of your mind forever? What if you got married one day, but if he knew the truth about the Thailand, the marriage never would have happened the way it did? Do you really want that?
Look. From the reading of your story. You made a huge mistake. You were drunk, and you asked some guy to come with you to clean the sand off your body. You were asking for one thing; but, taking your story as truth, when it started happening your logical mind overcame the alcohol and temptation and realized 1 night is not worth compromising a potential perpetual relationship.
You must tell him. If it makes you cry; Good, it's part of the healing process. He will be angry; he will be jealous; he will be hurt...but he has a right to know and its low down cold hearted and wrong to keep this from him. Don't try to tell me you're trying to "protect him" that's a lie to protect yourself. You are the one that messed up big time. "Should've said No" "It was a moment of weakness" "shoulda thought twice before you let it all go". Luckily, as I said, you stopped it. But you're better off facing the truth sooner than later or you'll spend the rest of your relationship wondering what would be if he knew the truth.