Masturbation [...] sure seems unlikely to have increased fitness—there is some “waste” involved.
I have no medical education, so the following are just guesses and half-remembered information from books...
Human sperm cells have a limited lifespan. From the fitness perspective, sex seems obviously better than masturbation, but assuming that sex is not an option for some reason temporarily, and the sperm cells would die anyway, the “waste” doesn’t matter. It might be even healthier to have them die outside the body. Which might be the evolutionary reason for “wet dreams”.
If the sperm cells towards the end of their lifespan have lower quality, masturbation between a period of abstinence and a period of frequent intercourse might improve the expected health of future children, eliminating the risk that the child will be conceived using an old sperm cell.
Some research of prostate cancer seems to suggest that men who ejaculate more frequently are less likely to develop prostate cancer. (The obvious questions—was this replicated successfuly? is it causality or mere correlation? -- sorry, I don’t know.) Regular orgasms seem to improve cardiovascular health and reduce the risk of heart attack. Again, if sex is not an option, masturbation may be healthier than abstinence.
I have no medical education, so the following are just guesses and half-remembered information from books...
Human sperm cells have a limited lifespan. From the fitness perspective, sex seems obviously better than masturbation, but assuming that sex is not an option for some reason temporarily, and the sperm cells would die anyway, the “waste” doesn’t matter. It might be even healthier to have them die outside the body. Which might be the evolutionary reason for “wet dreams”.
If the sperm cells towards the end of their lifespan have lower quality, masturbation between a period of abstinence and a period of frequent intercourse might improve the expected health of future children, eliminating the risk that the child will be conceived using an old sperm cell.
Some research of prostate cancer seems to suggest that men who ejaculate more frequently are less likely to develop prostate cancer. (The obvious questions—was this replicated successfuly? is it causality or mere correlation? -- sorry, I don’t know.) Regular orgasms seem to improve cardiovascular health and reduce the risk of heart attack. Again, if sex is not an option, masturbation may be healthier than abstinence.