Well, she was the one who was assembling it in the first place, she is in love with mc, so she does act like she is in the harem. Refusing to sleep with her won't change her attitude towards mc and won't bring too much dinstance between them, just like with any other girl. No outsiders here, just less interactions if you chose "friendship" route.
Some interactions were kind of mandatory in the first part, the second one will be a bit different.
Viewing post in Portal Heart Part Two [NSFW] comments
I'd say it was much more than just a few. I don't remember being able to avoid practically any sex scenes with her in the first part (except for the first one, and I can't recall if there were any more at the end). Honestly, between the femdom beginning (which at least you can avoid) and the lesbianism, I was really hoping I could avoid the scenes with her.