The board system is kind of geared to divide things by the type of sexual activities, although a story with lots of blood, guts, and gore would probably be best in [EX], regardless of sexual activity/character types/consensuality. [Trivia - Dawn of the Dead was originally given an 'X' rating by the MPAA in 1979, but Romero worked around that by releasing it 'Unrated'.]
We also have a 'Members Only Non-Adult' board, which would keep out minor viewers, if that sounds like it might fit your needs.