liz; I'm almost certain it is the guy's inability to perform on camera.
Couple that with the girl only agreeing to fuck said limp dick leads to this sort of disappointment.
The producers are pissed for sure but the chick is usually hot enough and they've spent good money (ALL they really care about), so they finish it off with the cocktail.
Oh, the ones that really piss me off are when they try to be sneaky with their editing and fool you; stop the scene, fill the girls mouth with the stuff, resume the scene and she lets it dribble out her mouth.
Please, we're not that stupid - well, I'm not
Just my opinion.