The look on Chad's face when she accuses him of cheating is very disarming, I wanted to read it as, "but I'm not seeing another girl" (because it's a guy), but really, seeing a girl or a guy behind a girlfriend's back is cheating, so maybe he really wasn't cheating. It doesn't explain the phone sex, but maybe it's just a job he has to earn money for the Tri-Alphas...
The fact that even if the mc rejects Sage at every opportunity she still wants to hang out more with him by the end of episode 5 gives me hope that he could start a relationship with her later (especially if he likes her but doesn't want to be a sleazy guy while she's still in a relationship).
If he's not in a relationship with her, he doesn't have to contact her when she's sick in ep6, whereas if he's in a relationship with her he must contact her. Either way, it means even if you aren't in a relationship with her, you can still comfort her when she's sick.