No it is actually a genuine and involuntary reflex. I guess if it's something you see in porn then I wouldn't be surprised if some of them are faking it, which I imagine would take some practice! But it's a real thing.
That's quite an impressive feat of willpower and bladder control. I sometimes have trouble getting the flow going standing fully clothed at the urinal because someone else comes and stand (also fully clothed) at the next one.
Those squirters you see in porn are actually pissing
A "squirter" just dribbles a bit of fluid when heavily stimulated, rather than sending jets of mimsy juice flying across the room.
These girls usually have some guy enthusiastically shoving something up them right until the second they release the floodgates.