At the risk of sounding like a broken record, or worse yet, dismissive of your concerns, the real harm here is not physical. It's all the worry, shame, and conflict attached to masturbation by so many men, women, and children, thanks to negative associations with all kinds of sex — done by oneself and with others.
Strong negative messages
These messages come from so many influential places – often for most of one's lifetime. It may not be true for you, but this cloud that hangs over many masturbators might send some looking for physical "harm" from this popular practice. It can be a way of justifying any bad feelings and beliefs about masturbation that might never allow going it alone totally okay.
'Blame everything on masturbation'
That said, your "blame-it-on-masturbation" reasoning is understandable. If one sprouts hair in "strange" places, sports dark circles under the eyes, and says goodbye to those always reliable erections during and after stretches of solo-sex, one could easily conclude that these unwanted reactions came on as a result of whacking off.
Then, could it not also be said that hair loss, far-sightedness, and love handles also owe it to masturbation? And what about greater self-confidence, career changes, and the urge to buy property? The point: is it possible that all of these occurrences stem from something besides genital stimulation? Might ageing also be the culprit?
It may not come up around the copier and coffee table, but many twenty- and thirtysomethings now include big black nose and ear hair plucking in their regular grooming routine. They'd also put down difficulty getting it up and keeping it there as a getting older symptom. Bags under the eyes? Yep, that's on the list, too.
How about an experiment?
If you're not convinced that masturbation doesn't play a direct role in the physical "consequences" that you're now experiencing, how about conducting an experiment? Masturbate less often, or refrain from doing it for a few months and see if your symptoms disappear.
Take note that not "relieving" yourself for an extended period of time may lead to other unwanted physical and psychological situations, including irritability, difficulty concentrating, insomnia, and wet dreams. Now, it is possible that any stress, guilt, disgust, etc. with your personal sex play is making it harder to get and stay hard – these feelings are frequently powerful obstructions to sexual arousal. Also, if you're masturbating past your bedtime, dark circles may stare back at you the next morning.