Voice over or dubbing isn't an easy task than what you guys would think it is and especially so if it's a pornographic film. Not only do you need to find an appropriate voice actor or actress for the job, you'd need to prepare a script for them and also to timed the recordings for each part due so that the film's lip moments tend to match with the dubs whatsoever... The worse would be the emotions and feelings to be taken to consideration when acting on certain parts...
The differences between dubbings and subbings would probably be obvious due to the fact where subs are only a translation of a certain language in the show into subtitles for us to understand... Whereas dubs would require much more extensive work to be done. Probably the worse type would be to dub in sexual contents where seiyuus would need to act and voice it out... perhaps a shocking video below would explain how its done...
The video below contains scenes of sexual contents so you may wanna refrain from watching if it does offend you in some ways or another...
Shocking as it is, that would probably be how its done... So its is truely amazing what voice actors been through training their voices for such. That's why I really respect them for their really hard work and especially so in animes where exaggerations and emotions can be felt from the actions...
Perhaps the best explanation would be to watch the anime titled Koe De Oshigoto! as it depicts how voice acting of such genres are difficult. Somehow or rather YouTube still have the first episode so you might wanna watch it...
