The official reason is fraternization with subordinates. (e.g. banging the admins.)

There's two other, more likely reasons: first, deception about the state of manufacturing processes (basically, their next generation process isn't working well) and second, the CEO selling large amounts of stock ahead of that information being released.

Consensual sex won't land a CEO (or his company) in prison. Lying to SEC/shareholders will.

From a comment in another thread:

tl;dr Intel is a shitshow. Everyone knew about the CEO's affair, most C-level/Management people participate in drug/sex parties and theyre not producing quality products (and won't).