Masturbation dislike comes from prudes. Edit: Wait, it's the whole can of worms that is sexual fantasy. Serious worm can.
Here's a place with articles that, if I remember correctly, take a sane and frank look at masturbation from a Biblical/Christian perspective.
Like many other Random Fundie Crap, it is actually a pretty modern thing. Basically, some dude wrote a book detailing how masturbation causes more health issues than snorting deep fried meth back in the late XVIII/early XIX (can't remember) century, it was EXTREMELY popular among pretty much everyone and spawned a cultural trend to frown on it.
Then religious folks
just so happened to find justification for that using the traditional fundie trick of arbitrarily patching together stuff from various unrelated parts of the Bible until it supports their conclusion (see:
Left Behind and similar crap).
With psychology and medicine advancing, the OMGEVUL side shrunk, but since fundies already had a totally legit guize quotes to back up the belief, and since fundies by definition almost do not care for anything that contradicts what they intepret their Bible quotes as, they stuck with it.
E: There kiiiiiinda is a semi-justification, in that, like Arx said, it might be interpreted to constitute adultery by some line about looking lustfully is committing adultery in your heart, but that is, in itself, problematic with things like Rule 34.