I think everyone is fully optional
except for the redhead (best friend). You can avoid having a
relationship with her, but you will still see somewhat sexual scenes with her, and them being flirty and creepy with each other at the very least.
I'll be honest, I have Lexi (the redhead) as my girlfriend, so I didn't get to see this when I played. But if you aren't in a relationship with Lexi, the MC creeps on her pretty hard anyways, and it's not avoidable. Lexi will still creep on the MC no matter what as well. With every
other girl (so far at least), they have been completely optional and you don't have to see any sort of sexual scenes with them.
So yeah, unless you plan on Lexi being in a relationship with you, either girlfriend or fwb, then prepare for the MC to still be a bit of a flirt and creep with her and vice versa. She is the only one so far that isn't truly avoidable.
Not a problem for me luckily.