Why I think Burke did something sexual up in that bedroom was the fact, 1) he did not belong in the room in the first place, 2) the voice heard coming up the stairs was very much like Burke’s, even before her mother had gotten the room. The voice said, “do it, do it, you bitch”, in a very Burke sounding voice. Remember, it is difficult to remember Burke sober, either in the book or the film, or this may be where the two paths diverge.
In any case, I believe there is a good reason to believe that Burke had nefarious plans for Reagan when he decided to go into her room.