There is no equivalent of the "F" word in Japanese, therefore any use of that word in an Anime or Game translation is mere "spicing up" by the translators.
Another explanation for the use of that word is that the Japanese writers written the word in English or Engrish.
While there Are a couple of Japanese words for 'Sex' and 'To Have Sex',those words are Never ever used as slang. Therefor the F-bomb is never said in Japanese media,unless it's said in English (or engrish).
Another factoid,wich is somehat related,Is that,these days,a lot of Japanese games and anime have a lot of english usage.
Yet Another Factoid: Japanese Subtitles Also have English in them. Also, Japanese Subbers (both amateur and pro) tend to crap up translations..,like their english counterparts,