I had one friend who wanted to keep his t-shirt on and wanted me to be clothed, at first. He also liked a dimly lit room, as did I.
After awhile, when we became fast friends, he would keep his t-shirt on, but sometimes would leave his shorts on, so I could remove them and have a good feel of his bottom while doing so. He also allowed me to remove my clothes as well, so I could be as comfortable as he was.
We had a medical office table in one of his bedrooms and I could position him many ways on it to perform various functions, (shaving his buttocks, give him enemas in various positions, etc. I could have him bend across the side of the table to get a good position to shave both cheeks evenly, and occasionally brush my erect penis against his butt. He didn't react to this at first, accepting it as a brief encounter.
One night, after shaving him, and giving him about five or six warm soapy enemas, followed by a warm plain salt water rinse enema, I led him to his bed, propped him face down over a couple of pillows and entered his lubed and opened butt from behind. He loved it and so did I. I must have cum for almost thirty seconds.
We had many such encounters after that night, whenever we could get together for a long evening. They were most memorable.