I knew a teen bodybuilder in high school. He did it solely because he loved touching his own muscles. I watched him masturbate at a sleep-over once: He caressed his biceps, pecs, nipples, abs, and legs like a lover. As he touched a new body area, he'd moan -- as if someone else were touching him. His hand worked his diamond-hard cock, his fist grasping the shaft so tightly you could see the veins in his arms standing out. He loved his own pecs and abs most of all, and powerfully ejaculated while his fingertips caressed his hard pecs and danced across his nipples. The semen landed on his neck, chest, arms, and belly, and there was so much of it that he had to remove the sheet from the bed.
He was completely disinterested in both boys and girls. Completely. He admitted to masturbating four times a day (morning, once at school, after school, and at night in bed), sometimes more. But not once did he ever evince interest in sex.