I'm assuming this means monogamous. Ulmir(a), Rune and Kazamsu are all strictly monogamous because of their personalities and circumstances.
Lirrahei is a bit more open to it if the player wants it, so you can have relationships with side ROs that are also open to poly (like the siren lady) while being with them. Ofc you don't have to though, you can just date Lirrahei alone.
The player can also choose to give Rune a condition that they'll only date them if its an open, poly relationship and Rune will technically accept but it will not end well for any other party in the long run 👀
How would the ROs react if they had spent a Night™️ (😏) with the MC and the next day someone walked up to them and said "Sounds like you guys had a good time last night 😏"?
Lmao this is a funny one to think about and yeah, I do answer react asks.
If the MC explicitly wants to keep it a secret then the ROs will either try to pretend it was someone else/didn't happen (Lirrahei & Kazamsu) or threaten them into changing subject/dropping it (Rune & Ulmir(a)).
Assuming this isn't the case and their relationship isn't official:
Lirrahei: Embarrased. They know that they tried to be careful and their embarrasement doubles when they think of how carried away they must have gotten despite that.
They'll either look to the MC to respond or sheepishly say yes then change the subject.
Ulmir(a): Finds it funny and is pleased that the MC may have had fun too, enough for an outsider to hear.
If MC isn't too scandalised, they'll proudly say something like "More than you can imagine," before redirecting the conversation. If MC is too scandalised they will (with a threatening smile) instantly try to use any dirt they have on the person to get them to stop talking about it or will forcefully change the conversation another way.
Kazamsu: Confused, and their expression reflects that. They're inexperienced in such matters and don't know what the other person is alluding to, plus Kazamsu would have been so caught up in the moment that they wouldn't have noticed any noises during it so wouldn't even know what the other person means by "Sounds".
MC would either have to respond for them or Kazamsu would confuse the other person into awkwardness and a 'nevermind'.
Rune's response is entirely dependent on the MC. As soon as the other person makes the comment, Rune will check MC's reaction and support/back them based on this the best way they can.
Rune is secretly delighted that people know MC is with them like that and a particularly unhinged Rune may try to use this situation to deter people and make sure everyone knows MC is unavailable (whether they really are or not).