- Masturbation removes old, damaged sperm that are passed their Use-by-Date from the reproductive tract. That would increase the fraction of healthy, speedy sperm, improving the male's chance of becoming a father. Notes biologist Jane Waterman (University of Central Florida), in her paper in the journal PLoS One, "In humans masturbating decreased the number of sperm a man delivered the next time he had sex but not the number of sperm that woman retained." She concluded that "masturbation is a male strategy to increase sperm fitness." Result: healthier and possibly more babies.
- Masturbation might be a form of advertising. According to this theory, males that engage in autoeroticism signal to possible mates as well as competitors how much they have to offer. Result: more mating, more babies, bigger families.
- Masturbation might be a sort of Victory Lap. Some animals masturbate after they mate. Since other members of the group know this, then this act signifies to other females looking for a mate that, "if he was good enough for her...." Result: more mating, more babies.
- Masturbation can serve a hygiene function. According to this idea, males engage in autoeroticism because it cleans the reproductive tract and reduces the chance of acquiring a sexually transmitted disease from a female he mated with and who had other recent partners. Result: a lower incidence of STDs, better sexual hygiene, more mating, more babies.
And what about females??? There have been far fewer observations of the females of various species. Instead, primatologists conclude, in the case of females the purpose is simply to produce enjoyable sensations...with or without batteries.
The Way I See It....all in all and across great and small, autoeroticism (at least among males) is a cornerstone of procreation and thus the formation of families. If religious fundamentalist views of the practice were to dominate, it would be a worrisome threat to family values and to our species. It seems that the church is always pushing for pain while males are always pulling for pleasure. Oh well...