Anyone notice this in hentai/doujin?

I've noticed recently that alot of female hentai/doujin characters exclaim or yell some kind of "no" right before they orgasm. Barring a manga where rape is the theme (such as the Yakiyama Line mangas), why is this the case? Is it perceived in Japanese culture that sex/orgasm is bad? Or that women are subservient and do not have to experience climax?

that's a fetish I think...

Most hentai is rape and so the girl getting raped dosnt want to like it. So she dosnt want to orgasum from it hence NOOOOOO!!!!! That my interperatation of it.

Here's the thing, the example that comes to mind is the Studio Wallaby/Wallabi Eva Doujin Asuka Ayanami Milk Cafe au Lait, the one where Asuak and Rei are 'in charge of sexual gratification duty' for that day. Asuka seems to be enjoying it or at least nonchalant about it, yet screams "No!" when she climaxes.

i thought its widely known that japanese have lots of sex issues.

one of them being women enjoying sex are considered lewd or otherwise perverted...though its quite normal of course.