"So.......if we accept the premise of this play, Gary Glitter should have been found "not guilty" in his first trial as his possession of kiddie-porn photos was likely to prevent him from carrying out real life attacks."
No.... because the porn in itself depicted unacceptable child abuse, and we follow the logic that the supply is (in part) linked with demand and therefore the consumer is an accessory to the abuse.
If Glitter's porn had consisted solely of Lisa Simpson being subjected to animated incest then his conviction might be more tenuous, since no harm was done to any living person.
And so it is in the scenario posed by The Nether - no harm has been done to a child, and no harm will be done to a child, because the entire meeting is a digitally-augmented roleplay game between consenting adults.
The question you should be asking yourself is that - in the knowledge that consenting adults do engage for instance in daddy/daughter role-play (apparently that's a thing) - is a digitally enhanced extension of that - entirely legal - activity ethically different?
"The basic premise of the plays argument is wrong: habituation to a certain practice, even if simulated, makes it more likely that the practice will be repeated in real life. Not less. Repetition breeds acceptance and familiarity: if it didn't, then what would be the point of aircraft / tank / battlefield simulators?"
It's not the premise of the play, it's the question the play is asking. In that respect, it cannot be "wrong".
Moreover, your references to simulators are hopelesly misplaced. Spending time in a simulator does breed familiarity, but does not mean that (e.g. if you simulate firing your tank cannon) you are more likely to do it in real life. In actuality it means you are more likely to use that cannon efficiently and effectively if you have need to use it. It doesn't mean that the moment you are deployed you will go blowing the place up firing shots everywhere. It simply means you'll be well equipped to use that equipment when you need it.
"If some deviant practices kiddie porn in private, its more likely to make him accept carrying out the real thing. Not less."
1. Cite your sources. Such claims require evidence. There is a whole section of criminal psychology which would disagree with you.
2. When did "porn" become a verb? What does "to practice porn" mean?*
* Don't answer, I don't want to know.
This play tries to create a discussion where no discussion is needed. In our society abuse of children is not acceptable.
Indeed it is not. But at no point in the play does it actually try to argue that abuse of children is acceptable. All activities are between consenting adults wearing digital costumes. People engage in roleplay now, so is roleplay with a more convincing costume ethically different? I'm inclined to accept that it might be. But not because of any argument that you have put forth in your drivellous rant.