This isn't about avoiding X-rated Web sites, but about dust and heat, which will wreak havoc on your computer's circuits. All electrical devices generate heat, which is why computers have fans inside or cases designed to boost air flow.
Heat is an enemy because it causes things to expand or respond differently to electricity.
If the printed circuits on your computer's motherboard and peripheral cards are too warm, they might make or break electrical connections. The usual symptom is the computer working perfectly for a short time after start-up, then becoming cranky and eventually comatose. But before you start thumbing the ads for a new machine, consider dust and the environment. Dust is everywhere. When it filters into the vents of a computer, it settles on components, building up insulation until the components don't give up heat as fast as they were designed to do.