You forget asuran nobles are ALL PERVERT to the Nth degree,the more serious a manga takes itself, the more goofy it is to see female characters written into these masculine warrior roles.
especially eris, the way she reacted to her deflowering is totally nonsensical, contrary to human nature.
We also get a bit of her own perspective on the matter and
she hated herself for it cause during the act she realized rudy was much more younger than her and she felt like she was abusing him since he seems so mature