You can currently have sex with Nia in the love route, but not the slave route. As for why you can't in other areas? This game is still in development.
Continue her story, it's her second to last story event in the love route.
https://harem-hotel.fandom.com/wiki/Nia
By that logic, you can do a threesome with Autumn and Autumn, and Felicity and Emma too, you can have sex with Kali in a library, you can lead Maria on a leash in public in a hospital, etc etc. You just need to play the whole goddamn game again if you want to see it again, since it's a one-time only, with no way shape or form for repetition.
By that logic also, you have hundreds of thousands of unique events with each character, since every single frame you see is different.
Do you understand why this is a dishonest counting method in this type of game? Or are you blinded by being a fanboytoy?
Love the game (even saw your ending to current version thanking everyone who played to reach it) kinda sad that it seems I'm unable to articulate myself well enough that you at least look into the issue I presented you with (if I understood your comment "The game is coded to look for instances where characters have so done things in other events and those things will be brought up if the right conditions are met") To mean that I should have expected the results that occurred instead of looking at the illogical situation of acting like we are all friends and know a lot about each other (enough to go bowling, get drunk and.... ) one minute to meeting each other for the first time the next.