Well, throuples are supposed to work that way too, I mean the 3 in that throuple are together but interdependant, and can have fun with each other whenever they want, so girls have to be BI if they want a MFF throuple to work, same thing with guys in a MMF throuple.
Throuples are, essentially, 'sharing'.
Vic is supremely open-minded and definitely BI, Mila doesn't seem to be though, she really seems to be a solo, there are hints that Nami wouldn't mind exploring bisexuality when with Vanessa, and that makeout with Vic, damn... other than Bella, not sure she'd want to be in a throuple though, she really seem to want to be solo with the MC.