That, and IMO, when someone uses sex to sell a game, more often than not, it's very shallow and terrible. Now a good game with a sex scene in it, like Mass Effect or something, that's probably a different story. How you would actually come up with a good wording in guidelines for this tho.... I dunno. They might say "case by case basis" or something.
as I suspected this is steam going around targeting games whose main focus is sex, not "Games with sex in it", because there's way too many of those floating around and the definition is too broad
but when the focus is naked anime women, then I think it's safe to say that it's probably not a good idea for those devs to put all their eggs in one basket