I would say this is the weirdest thing I've seen, but I've seen one erog game where all the characters except you are female versions of various historic figures like Oda Nobunaga and Napoleon. (Granted it plays like some weird bastard child of Fire Emblem and Nintendo Wars)
I may be wrong, but I don't believe that Fate/Grand Order is an eroge. For one thing, as far as I know, there isn't a single sex scene in the game, which is... kind of required for it to be an eroge, by definition.
Eroge or not, I'm sure some people still get off on it.
If we're going by that logic, why don't we just dump every game from Japan ever, plus Borderlands, Skullgirls, Street Fighter, and just about any other video game into a bin marked "sex games"? People get off on those too, you know.
EDIT: Oh, and Overwatch too. And league of legends, I've seen a ton of porn of League of Legends.