I really wanted to like this a lot more than I did, because (breasts aside) there are some really funny moments. Poor hapless Yuuma is the son of a light novel author - the copy for his dad's book Oops! I Went to Another World and Accidentally Saved It! reads "Over a million copies in print somehow!" That's bad enough without a princess and a demon suddenly showing up and proclaiming him both hero and demon lord...and that turns out to be because his dad's books are true and based on his own isekai experiences. (You can guess who Mom is at this point, I'm sure.) Add in a childhood friend who manages to miss all of the sexy shenanigans and just wants to be friends with the demon and a grandfather who is So Over This Crap and it's a pretty funny story.
But.
The rest of the humor relies on the ladies sexually assaulting Yuuma. That's never funny in my book. You had so much potential, book. Why did you squander it?