Well that's a tricky question. Really, here in America, and most other countries, it falls under the interpretation of obscenity laws.
The only things that I know cannot be charged obscene is a woman's breasts (supreme court of our country has ruled that breasts, which we use to feed our babies, can never be deemed obscene, I believe this came from a case when people were freaking out about women breastfeeding in public) And, anything that has artistic value (which is once again subjective).
As you can see this leaves 99% of adult entertainment subject to obscenity charges. We really have very little protection as far as first amendment rights. We are at the mercy of whatever asshole prosecuter deems obscene when it comes down to it. Fisting could be prosecuted in the same way as any penetration could.
Fisting may be setting itself up differently only because if they do come after us they are going to start with the more obscure "harder core" stuff. Stuff that they themselves might not do in their bedrooms. They probably see it as an easier court victory. It would be harder to prove sexual penetration from man to woman only because we all do it (heterosexuals anyhow).
I'm not just concerned with fisting..I'm much more concerned with my BDSM stuff. I think they could convince a square jury that any of the above is "obscene" because it's a niche and not as prevalent in adult bedrooms as "regular" sex.
The bottom line is, you need to think about where you live, how conservative the majority is and play the odds game. All they need is 12 squares to convict you. That's not hard to find in any country LOL.
Even though all the laws are subjective and open to interpretation, that helps us nil because it is the prosecuters that choose to indict and they can always find 12 holier than thou jurors to convict you if they want to. Our ideas as to what is obscene has no bearing. Very one sided.
