Can't say much from a medical perspective but the Gauge scene in Weapons of Ass destruction is a good example of the piss/gush dialectic. Once the double anal starts and she moves off to whatever dead-eyed fantasyland she seeks solace and security in, she starts gushing all over the place and is quite freaked out once it starts.

If it is a 'piss scene' it sure isn't played out like one. The guys just keep jack hammering away without missing a beat and Gauge goes back to her happy place thinking about her next bottle of Wild Turkey/slam of H.

I'm not sure if this helps out any, but I like thinking about Gauge getting fucked in the ass by two big cocks, and I hope you do too.