To me, it doesn't feel like this is something these characters would do, at least not openly.
With Sage, it feels like the best track for her going forward would be to just continue the FWB arrangement, even if she's starting to feel something more for the MC. Remember that she's a 4th year and will be graduating at the end of the current academic year, so she'll likely be heading off somewhere far from B&R and in order to avoid a painful break-up or likely unsuccessful long distance relationship, she may not want to start a proper relationship with the MC to spare herself more heartbreak. She still might feel those feelings of jealousy or suspicion when she sees how he is with other girls and they with him, but she'll probably bottle that up since we know from Ep 6 that this is something of a family trait.
With Quinn, she very much hides how she really feels under this mask of non-chalant bitchiness, so she too wouldn't actively show any jealousy and in some ways I can see her at least convincing herself that she doesn't care about him being with other girls or other girls being interested in him, but deep down she might be even though she won't show it.