Looking at the photo (multiple small dark/brownish pinpoint spots scattered over the shaft skin, no swelling, no discharge, foreskin looks normal) and going by what you've described (no itching, no pain, nothing else) - I can't give you a certain diagnosis just from a picture, but here's what this pattern most likely represents, and what it's less likely to be:

There's no medical evidence that masturbation itself (regular or vigorous) causes a "disease" - but forceful or excessive friction can absolutely cause this kind of superficial skin change (small bruised/pigmented spots) without any pain or itching, because it's just skin/vessel trauma healing, not an infection. If that's the likely cause, easing off the friction/pressure and using a lubricant during masturbation usually lets these fade over a few weeks.

Since a photo can't fully substitute for a physical exam (texture, whether spots blanch when pressed, and whether they're flat or raised all matter for the diagnosis), if the spots don't fade in 3-4 weeks, change in size/color, start bleeding, or you develop any new symptoms, it's worth a quick in-person look by a dermatologist or urologist. Otherwise, this pattern (asymptomatic, no itch, no pain) points toward a benign, self-resolving friction-related change rather than any sexually transmitted infection or serious disease.