Cards Against Humanity; is it a game to play amongst professionals (on or off duty)?

This is not a "mixed setting" game. It is one of adult humor, to be played with "blouses off" so to speak. Much like hearts, spades, or most other downtime games with no stakes involved.

As a civilian, I would never play it in front of my boss, my juniors, nor my HR rep, but I would play it among friends "who could take a joke." It is not for the easily offended.

That said, were I still in the service, this is something that would be played in the barracks room, not in the squadbay, if that makes sense. It being played isn't the issue, the fact that you saw it being played was. A casual observer could walk in, and see "good clean fun" with no ill will, and things could quickly escalate out of control.

My personal recommendation, were I the witness would be to take it to a more private setting, so that perception didn't bite them in the tailfeathers.

The game is offensive and inappropriate, and that's the only reason to play it. Guys in my platoon played it a lot in Afghanistan. I never played, but I certainly wouldn't think of telling somebody else they couldn't, or shouldn't.

I agree, it is not appropriate for a professional or public setting. I have played it with other service-members, although it was at a going away party for a buddy who was deploying in a private setting. It is not something I would want to see in my day room, armory, or barracks. However if a Soldier chooses to play the game with close friends at his home I don't see an issue with it. Those involved should know going in that the game is offensive.