I'm going to be blunt here. If you're playing an ADULT visual novel and not expecting the girls to be in sexy situation(even random ones with no context),then you're playing the wrong kind of VN.I do wind up liking the characters and plot a lot more than the sexy bits,but at the end of day the goal is to wind up in sexy situations.
To me, it wasn't Penny being OOC,more so than her revealing a side of herself she obviously wouldn't share with just anybody. if you had an issue with how it was done,i would get that. but it just seems like you have issues with the whole thing entirely and her having a kink at all is a betrayal of her character. I also don't get how you would see it as OOC considering that up to that point,we didn't know much about Penny at all,much like all of the other girls.
The story is still just starting and it's reasonable we wouldn't know everything about the girls and that's not even getting into how they could develop later on, both in ways that are sex-related and not.Most of these girls are still teenagers who are discovering their sexuality after all. Cari is a good writer who I trust can pull that off and have sexy scenes that make sense with the characters,even if the context might not be there.
So some of the girls might turn out to have kinky sides, which is fine by me,since I find vanilla sex on its own to be boring. But,on the flipside, I doubt that all of the girls would be that way and some might be vanilla. Having them be all one way or the others would make all of them samey, which for an AVN with multiple LIs isn't something you want.
PS. It seems to me like you like mostly vanilla stuff,so if you haven't heard of it already, Lust Age is probably the perfect AVN for you as it's vanilla as it comes and the weirder kinks can be avoided.