Dear Miss Manners...

The other day I was looking at magazines at a local newsstand, trying to decide between Newsweek and Time.  I looked up and saw a gentleman slip behind the beaded curtain that separates the Perv Nook from the rest of the reading material.  I realized, to my horror, that it was a co-worker, out to score some hard-core porn on his lunch hour.  I put my head down and left the store quickly.

What is the etiquette in such a situation?  Should I have greeted him?  How can I ever shake his hand again without wondering where it's been?  Is it wrong to expect colleagues to buy their porn on the way home instead of 2 blocks from the office?

Just wondering