And fantasies are not logical, except in the way that they totally are: Fantasy!Daniel is invulnerable and Jack can't "break him with tenderness", while Real!Daniel is wonderful but he's also a real human being who can be hurt by the world, and that's terrifying -- especially for someone like Jack who is always focused on protecting the people he loves (including protecting them from himself).

There's a good book called "Arousal" by Michael Bader which theorizes that sexual fantasy is all about getting the lizard brain whatever it needs to feel safe in order to get off.