There is a bit of leeway with the F-bomb. The rules technically say as follows:
Kor
Source (pdf file): http://filmratings.com/content/downloads/rating_rules.pdfMPAA Classification and Rating Rules said:A motion picture’s single use of one of the harsher sexually-derived words, though only as an expletive, initially requires at least a PG-13 rating. More than one such expletive requires an R rating, as must even one of those words used in a sexual context. The Rating Board nevertheless may rate such a motion picture PG-13 if, based on a special vote by a two-thirds majority, the Raters feel that most American parents would believe that a PG-13 rating is appropriate because of the context or manner in which the words are used or because the use of those words in the motion picture is inconspicuous.
Kor