Seems like a pretty big difference. It's got to be worse to actually do something to somone in real life than not do that.
Not really, otherwise perpetrators will just "I was just looking at it, I didn't do anything as bad as creating it". Their act is still illegal.
There was a cartoon picture I remember seeing around 15+ years ago of Bart Simpson performing a sex act. In some jurisdictions (such as Australia), this falls under the legal definition.
Just because there are different degrees of severity and different ways to offend doesn't make it not contraband.