Perhaps I'm missing something obvious, but I was sort of surprised to find that there are somewhere over 600 picture matching a search "sex -rating:e". Are there any cases where this is actually a legitimate setting, since the edit page points out that "Explicit tags include any sexual body parts and acts"?

I have suspicions that some of this is due to tag implications--for example, something without any actual sexual activity going on gets tagged threesome instead of trio, the former of which implies group_sex, which in turn implies sex.

Partially. There are some mistags such as in post #670775 which got tagged sandwich_position, and sometimes a tag like threesome gets added mistakingly but whoever noticed it first didn't remove sex. Sex, like genitalia, should never be tagged on rating:s. There are some in rating:q that sex might apply to but it's rare as they usually need the tag removed or rating changed. I'll take a poke at them to see what I find.

It's another of those hard-to-define things. Some kinds of sex just don't seem that explicit, especially if used for humor:

post #230094 post #281706

And then there's images that may or may not contain sex. Those tend to get rated questionable and tagged as sex. Should be one or the other, but sometimes it's difficult to tell which one would be more appropriate. Example:

post #207897

Facial expressions say sex, but it's hard to tell.

There definitely shouldn't be any under sex rating:safe. But there often is, and it needs frequent cleaning..