Chika is literally setting up a threesome, and letting Sensei choose another girl that she'll talk to for him so that he can have sex with her. I don't think she's that prideful or jealous of other girls, when it comes to her man, and is just territorial. As long as she knows what's going on, I don't think she'd be that bothered by whatever he's doing. She's probably the most Harem Positive girl besides Miku.
As for "paying her phone bill", what I was implying is that Sensei is making her life notably easier and more stable. Thanks to Sensei she is able to get a hold of her sick sister as well as her delinquent best friend, etc, in case anything goes wrong, and can look up whatever she needs to. Having a phone is actually quite a big deal, and having more money is also quite a big deal. Especially for someone in Chika's situation. I don't see her actually giving up Sensei over him cheating on her, but I can see her being upset over it and maybe doing something she'd regret. It's the broken trust that'd really hurt her imo.
Btw, being a sex slave and being apart of a Harem aren't the same thing. A Harem is simply a group devoted to an individual. Most of the class might as well be an unofficial Harem already. Miku is surprisingly the biggest supporter of the Harem imo. She's more or less been saying Sensei should make everyone happy by fingering them since he fingered her.
Also, unfortunately, If the gods break them, they'd probably become that god's sex toys rather than apart of Sensei's Harem. Some being has already gone after Sana, and HOPE makes it clear they want to devour all that Sensei loves.