@wundayatta: possibly because they’re old, frail, infirm, mutated, or just plain shaped funny. Evolutionary reasons for this abound. The simplest explanation is that the male body is constantly producing sperm, and they simply have limited lifespans (hard to argue with).
Then there’s the possibility (I would argue likelihood) that different sperm have different functions, and not all are there to fertilize. More controversial reasons for why that is are discussed by various biologists who go into this in varying detail (Robin Baker’s Sperm Wars for instance) – I actually asked a question about sperm competition the other day.
And then the consequences might be controversial for varying reasons. For instance, masturbation may be partly about shedding old soldiers to make room for new ones. And masturbation may be stigmatized because it’s to every guy’s advantage to taunt masturbators while masturbating himself.
Either way, the whole matter is a rapidly changing field of study.