I have a belief similar to firball5's. These whimpers shows that the girls in these films are weak (but aroused) when participating a sexual activity. Obviously, these moans and whimpers are appreciated by Japanese audiences. That's why actresses moan, whimper, and even scream like that, and I don't think real-life Japanese girls do that during an intercourse.
My belief is since the men are dominate in Japanese society the whimper is an easy way for a woman to show she is pleasing the man and he is pleasing her (whether she is being pleased or not). It lest both the man and the woman save face regarding their sexual prowis.