I don't think sexing with real boys is something which God approves.
I beleive there is a difference between things we desire and things we actually go after.
I desire boy sexing... but I would never go after it in real life.

How then do I handle the desire?
Ignore it... don't think I can.
Eridicate it... don't think I can.
Manage it with the best Holy Spirit empowered self-control possible... doable.

How fantasy and masturbation fit into this third option is my ongoing exploration.

I think your question is a brilliant one. But it's implications are vast.
If we say that any fantasy about stuff that would be unholy if we actually did it was wrong... then pretty much throw out every fiction book and movie that has been produced in modern times. Is that really what God expects of us.... if so... I know that I for one am miles away from such holiness, as are most Christians I know.

Blessings
Cat.