A friend of mine just told me why men never make a noise in porn films; it refers back to Miss Kitty's point about the male porn stars being paid more than the women.
When the sex scenes are being filmed the director the rest of the crew are making too much background noise for the real noises the actors are making to be any use if they are taped. So the groans and moans are dubbed on afterwards and whilst the companies are willing to pay the reduced rates for the women to come back and moan they are not willing to pay the extra money for the men to come back and groan.
I don't know whether my friend is right but it does make sense.