A harem is when the girls are all knowingly and willingly in a relationship with MC at the same time. If not, then MC is just cheating on them all.
In order for harem's to make sense, the story needs to be based around it. Like WVM, My New Family, Ecchi Sensei etc.
The story in this game, despite MC being unaffected by cheating so far (mostly), doesn't make sense for that harem. The girls literally say they don't want to, and even if they change their minds, it'll be a limited 2-3 girl "harem" not a 10 girl harem.
Take Katie/Debbie and Lily, that is the most obvious one. Katie wouldn't view that as a harem, but a fun thing to do, as she has a lesbian stat and will likely want to act on that eventually. If Debbie and Lily start dating, and they fool around with Katie/MC, then it would be swinging, not a harem. They're in separate relationships.
Now take Jenna/Jolina, the next most obvious one. Jenna seems to be interested by it, Jolina was likely in a lesbian relationship so she could be interested in a 3-way FWB thing with MC/Jenna eventually, but probably not a 3-way relationship.
As for Monica, the only plausible one is with Zarah, and the only reason that is plausible is because they are best friends and Zarah isn't interested in a relationship. She would be fine as a sex buddy and perhaps an occasional 3-way, if Monica would be ok with it.
As for Katie/Jenna/Monica, I can't see it. Maybe Katie/Jenna if Katie's lesbian stat is really high, but I can't see Monica doing that. She'd probably murder MC if he was banging all 3 of them at the same time. She has that suspicion stat for a reason.
The other plausible ones are Wanda/Angel and maybe Bella and Elaine would be interested, but not with each other.