Many moons ago I worked for Energy; I only had a Guest clearance but still had to go through training on the security structure and protocols so I'm at least sort of aware of how it works.
The "government computer" he moved his porn stash to was 100% his non-secure office machine. I'd wager it was connected to ESNet, Energy's ultra-high-speed global network designed to allow researchers to share massive amounts of data quickly and easily
but not securely.
The secure stuff is on a dedicated, contained network akin to JWICS, a totally isolated and hardened system. Terminals are locked down tighter than a Supermax; there is no way you're uploading personal data from one of them. I'm sure there's DUers who have TS/SCI clearances and understand you can't just plug a flash drive into a secure box, the Q stuff (DoE's highest security level) is the same if not tighter.
He lost his clearance for being spectacularly inappropriate and careless, and I'm totally on board with that, but he wasn't putting XXX pics in the same directory as W-80 blueprints.