Ahh it's one of the maddest scene in some Henry Miller novel I read years ago. Problem is, it's great to read about it, it's probably amazing to do it, bu how to show it? Kinda of like creampie I guess.
I guess you could have the male actor scream out "I'm pissing!" and the actress screaming something in response. And of course she has to eject the piss on screen. And then we all come back here and debate how real it is until kingdom come.
I am good with anything anal as long as I don't see any "chocolate chips".