I agree, there are few, if any, significant differences between the physiological effects of masturbation as opposed to intercourse.
Many of the negative health attributions to masturbation probably have some origin in the founding beliefs of Chinese medicine. Most traditional Chinese medical texts state that a man should not ejaculate more than roughly four times a year. It is believed that to do so excessively can leads to a specific energetic deficiency in the kidneys, leading to bodily weakness, as well as other potential systemic disruptions of "chi."
Masturbation is probably more specifically singled out, not only because, due to relative ease of time and availablility, one may be more apt to engage in such with slightly greater frequency, particularly outside a relationship -- but as well, once you delve into the intricacies of Tantric sexuality and the exchange of vital energies or essences, it is stated that the "lifeforce" gleaned from one's partner serves generally to counteract or surpass the energies lost with the expulsion of semen, in many cases.
George Carlin once joked that if a man masturbated so many times in a day, he would pass out squeezing the door handle on the way out of the house. Thus far though, there have been no such documented cases.
