In my experience artists usually add it where it would be 'realistic.'
So, it seems to come up most often with huge dicks, which are then most prevalent in demons, rape, loli, and futanari. Then when you combine all four, you get askray.
She might have a prostate ^_^ its just below the bladder. and everyone knows only 20% of semen is sperm anyway (Oh God I'm biologically theorizing about futanari...kill me now)