Some characters may be open to a poly relationship. Others are monogamous. Others might be up for the occasional no strings attached threesome.
To elaborate a bit...
Tahir is monogamous but since he's so ridiculously head over heels in love with you and desperately wants to please you, he'll probably be okay with threesomes...for a while. Note that this isn't his preference though. And who knows the long term implications.
Kadijah's monogamous and possessive. (Obviously.)
Aislinn...depends on how much she's had to drink.
Zilv is pretty chill all around and is cool with most things. He's probably the least insecure party member.
Astrid isn't generally interested in sleeping around with other people, though she could be curious about people who hypothetically empathize with or understand her condition. Also she thinks Tahir is kind of cute and wouldn't necessarily mind railing him with you once or twice...
Egyptrap is open to a threeway relationship with secret love interest or just solo with you. Same goes for secret love interest.
Sonya is an alchemist/scientist first and foremost so collecting data and experimenting is a big deal to her. That being said, it'll be hard to crack the logical exterior and get into deep emotional territory with her and who knows how that will affect her view of relationships.
As for side character flings, I generally do it on a case by case basis. Maybe out of sight out of mind unless otherwise noted. Like if I'm like "this will definitely alter your relationship with your love interest" it will do so. ALSO KADIJAH ALWAYS KNOWS