For Riona, this moment was most likely a culture shock. She never got involved with a normal relationship and it seems that everyone looked at her as nothing more than an object (from this her self-esteem problems).
Suddenly, a man appears who is polite to her, although he uses her sexual services, but sees a personality in Riona in the first place.
The storyline of MC-Rion's relationship is very well written and develops logically, one of those arcs that are in my top.
It seems that for the first time in Riona life she begins to experience a romantic interest, this can be seen from the scene on the couch during the party, and this is most likely why DPC didn't go with just another sex scene in Ep.9.