A reference to the anus, alluding to the fact that it is the place where one defecates from, and has a star type shape to it.
Chocolate refers to the fecal matter color, that is generally expelled from the anus.
Starfish refers to the star like shape of the muscle that radiates from the hole.
Generally used in a more humorous vein.
Can be used in a derogatory sense, by heterosexuals in reference to homosexual males, and their desire for anal sex.