Can too much masturbation cause thinning of hair?
Maybe, but there is absolutely no evidence to suggest that masturbating is at all bad for you. I only say maybe to the extent that maybe it also causes cancer, or increases the chances of your testicles swelling to the size of grapefruits. Hair loss will be far far more influenced by genetic factors, e.g. is your dad bald? (or maybe I suppose your genes influence your propensity to masturbate, which then causes hair loss, who knew!)
This is an old wives tale to say the least. From what I understand, when a man ejaculates, the body then promotes testosterone/sperm production. Excess testosterone is supposed to aid in hair loss. And though raised levels of DHT causes hair loss, masturbation only increases it ever so slightly. Unless you’re pulling your own hair during your fantasies while masturbating, then masturbation will not be a factor in the loss of your hair.