When I was younger, and was a little upset, and would sigh. I would usually sigh the word, "Dang...". Whereas many people today, will sigh the word "Fuuuuuuck" (like that). This is ridiculous. The word should be used only for times of great distress, such as "Fuck! I just burned my hand!", NOT "Fuuuuck I'm so bored."
With enough time, the meaning and sensitivity of words can change. Example: Gay. Years ago it used to mean being happy, or carefree. Now it is associated with homosexuality. Rarely do you see it used in it's original context, such as if you go watch some old plays. The latest meaning, seems like a taboo topic for millions of people, due the reference to one's sexuality. I may possibly be stretching this, but, you could say that the sensitivity of the word itself increase, no?
See the difference? The second group are a group of de-sensitized people. The word "Fuck" is present in nearly every porn video. Thus, registering to the head and desensitizing the person.
This is essentially why Porn viewers are less sensitive (and my rant on "Fuck").
Way more than half the doujins I have read, have used the word "Fuck" in it, during the sex. I don't want to exaggerate the average number on those pages, but there tends to be quite a few in there. I haven't watched a hentai video in quite some years now so I don't recall if there was much usage there.
Another reason is that Hentai usually have deeper stories, thus making you feel for the character, instead of trying to physically feel them.
So I guess they should make pornos with deep stories and use the word "Fuck" less, so that the audience can feel the characters.
This would require porn actors, that you know...can act. And people to come up with a decent story.