I think it's immoral, but in a relatively benign way; I'm not going to actively comdemn anyone for it. The only person you're potentially hurting is yourself (and your partner/future partner). I'd rather people did it than slept around, though.
But how are you even hurting yourself? On what basis is there any physical or emotional 'hurt' involved? At most it could be considered hurtful if done to excess, but that is the same with numerous other things from eating to exercise. It's a healthy sexual expression (as a rule I tend not to take any moral advice on sex, relationships, marriage or children from an organisation with a male hierarchy that never experiences any of these things), and is only considered 'wrong' due to some misplaced guilt.
It's extremely easy to get addicted to, in the full sense of that word, that like alcohol it can run your life. Additionally, it desensitizes you to a lot of sexual things when done over an extended period of time. It also dilutes your brains ability to make proper oxytocin pairings through overstimulation, especially when used with porn.
What utter rot.
I like to do it without finishing, so there isn't any oxycodone involved anyway - just a passtime, like playing Risk online or Freecell when the net connection's down.
But as to the question posed by the (on this issue) sensible Afleitch, it can cause a slight malformation of your rutabaga, if done for more than four hours at a time. In my experience this is always set right betimes the next night's boomsing, however.