LOL! Well it wouldn't have even occurred to me that it is x-rated (hey I'm from the mid-west where the farm animals do their thing in public - lol). But come on, not appropriate at work, or kids? Listen to the cartoons (even in my time) with all the 'over-their-head' comments it's amazingly unappropriated - but it's fine if it's never going to be understood.
So IMHO I wouldn't use a safe filter or caution comments unless someone actually complained. You don't want to offend anyone, but at the same time you want the work seen. You have logical reasons for assuming this wouldn't be offensive to people and not picked up on by kids, so I wouldn't do this till you actually have reason to think it's a problem (like a complaint).
I agree it's a cool image.
-- mary ellen anderson