It's not very nice to be a sperm. Sure, you get a cool little tail to whip around, but other than that the job description includes nothing but misery and a painful death. Not to mention your workplace, being inside someone's testicles, is swarming with millions and millions of co workers which you're bound to hate and want dead.
Don't worry, they do die painfully everyday. But the catch is, so do you.
If your host masturbates, you will be exposed to the outside world and burned by the light like the vampire you wish you were. If he is sucked off, you will descend into his lover's stomach and be eaten alive by burning acids.
Of course, you have a one in a million chance of becoming a human being (a fate worse than all of the above) if you're not burned by vaginal acids, but you can read up on that somewhere else. I'm not a doctor, you know.
