Well, I worked at a law firm where if the attorneys held a secret santa amongst themselves, I can totally see it being "adult" themed. The head partner and the "managing" attorney were both female too so it's not like it's an old boys club thing. Just very tightly knit group.
In high school, one class of mine held a secret santa. I had a legit gift, but wrapped up a condom (new/unopened) in a small box that normally holds jewelry as a gag gift. My buddy didn't have a gift so I gave him the gag one. That one caused a funny scene at my sheltered high school.
On a more serious note, my network group just had a secret santa thing. Alcohol was by far the most popular gift passed around.
Oh, I thought dirty santa was just that...dirty. I guess I'd have gotten myself into trouble.
My first thought was a "Weener Kleener" soap... but I guess that was taking the Dirty Santa thing toooooo literally.
What about a Snuggie? You know everyone wants one, but never ever would admit it.