What are the negative effects of masturbation? (males)?

Nah, this is a common masturbation myth. You won't get weaker (or stronger, alas) by masturbating. The only negative effect I can think of is becoming addicted to it in an unhealthy way; being obsessed over it. As long as it doesn't control you, you should be fine. Aside from the amazing feeling,there aren't really any good effects... I suppose you could improve stamina if you did certain exercises... but where's the fun in that?