are all sex scenes are optional ? +
The main characters have sex with others other than mc ?
like sex scene (rosalind * killian)
Regarding your second question: This game is about a particularly perverted high class brothel in which men use women freely in front of everyone. I think there is no way to prevent seeing almost all women have sex with other men (and women). In fact, the game opens with the MC watching pornographic footage of his mother that is an important backstory element because he found it accidentally when he was a child. I think so far you can probably avoid seeing Hana, Mina or Kathleen having sex with people other than the MC; though Mina is initially Killian's girlfriend. I think there is no sex scene involving Sophia so far. Regarding specifically Rosalind and Killian: Both are the MC's natural girlfriends, either individually or both at the same time. But at least generally speaking you can avoid both. Same for the three carnations. You can pick and choose which of them, if any, you have a relation with. The game branches very heavily. But them having sex in public with
someone (which sometimes may optionally be you) is obviously the point of this game.
Regarding your first question: There are two potential issues here. Many games have a main love interest that you cannot avoid. The main character will love her and have sex with her, and it does not occur to the developer that someone might not want this. I think this issue does not exist here.
However, while the MC can so far always opt out of having sex in public with anyone (I think), there is at least one scene where Kathleen asks the MC to her office, injects him a substance against his will, and forces him to have sex with other women. Here the player cannot avoid this because it is part of the story that the MC cannot avoid it. There is also a scene where the MC has sex with a prostitute to impress a client. Not sure if you can avoid this, but if you can avoid it and insist on doing so, I guess you will seriously restrict the directions in which you can take the story. There may be more like that. Plus, as I said, you cannot completely avoid seeing other people having sex in public or at least in front of the MC.
I think to some limited extent you can avoid even seeing the exhibitions, but then what's the point of the game? If you are squeamish about any of these things, this game is probably not a good match for you. But then it's also a bit of a mystery why you are interested in it in the first place.