Yeah, that's much harder to define than people generally assume. Cleavage, for example, is generally perceived as sexy, but seldomly as pornographic.

In general, in our Western society, the whole breast seems to be fine, as long as the nipple is covered up. So, what they could do is show you nipples, if you actually search "nipple". But maybe you're looking for male nipples, which obviously are not pornographic at all, so that doesn't work either.

"Vagina" and "penis" have enough educational material behind them that that may actually be what you're searching for.

"Pussy" could be a cat, "dick" could be a Richard.

Which is another aspect, making this harder. Our society loves using non-sexual words to refer to sexual things.

I suppose, if you actually throw in the word "porn", there's hardly ever going to be a scenario where you were not looking for pornographic content, but it's not entirely impossible either.

A nice example is the bird called "great tit" (https://en.wikipedia.org/wiki/Great_tit) - "tits" being one of the classic "pornographic words" (e.g. 1972 "Seven Words You Can Never Say on Television"), so when someone is searching for "great tits", is it reasonable to assume that they want dirty pictures or should the search engine return just the birds?

If you search for girls online are you typically looking for clothed ones? That's a rational assumption but many searching for girls are not looking for the same thing as you might want.