NTR is a fairly normal thing in porn. The problem arises when the man who's wife/girlfriend gets turned on when he watches himself getting cucked.
Scat, incest, vore, gore and snuff are way worse things in hentai.
Why would he want to now? He has a harem of his own, and raping the princess would probably fuck things up.
He could still have a horde of goblins or whatever have their way with her.