A heterosexual individual who derives sexual pleasure during the act of excretion. This is not the same as an individual with coprophilia, whereby arousal is produced by feces. The fag shitter is aroused by the feeling of excrement escaping from the anus, as if it where a phallus being pulled out.
Mark: I moan when I shit.
Matt: Mark, you are a fag shitter. Look at you, deriving sexual pleasure from your own excrement. You silly boy.