All of this ignores Sage's personality. If Sage were like this she would have broken up with Chad the minute she found the panties in his gym bag. Instead she hung on to the idea of finding out who Chad's bitch was. Sage is not the type to just let go. The subject of Zoey is not over and done with.
I've seen that you have trouble understanding this and there really is no point in discussing this with you when you misunderstand the characters on such a fundamental level.
So because Sage found panties in Chad's gym bag...she's hung up on the idea of finding out who Chad's bitch was...
...and as such, Sage is not the type to let it go?
And you're comparing this to Zoey?
1. MC's old girlfriend is Zoey. Tada! Sage has found out! Mystery solved. Comparison finished. MC wasn't scared about telling Sage because there is no conflict. There is no romantic relationship with Zoey. She's just an old friend to the MC.
2. She's talked to him about it said reveal. She was scared, and now she isn't.
3. Her issues with it are resolved. She trusts him.
Problem solved. Combined with the MC dealing with Zoey, choosing Sage, and even having a potential ILY both ways in the process.
While it's possible I don't understand Sage as a character as well as you do, you don't seem understand the comparison, or the narrative, as it has shown us.
Your reluctance to talk about it only shows me your assessment is wrong, and can't admit to it. In no way am I stretching the narrative. That's why I went and posted screen caps showing that Sage was only concerned about being cheated on, like she was with Chad. She was scared of that. The MC sees Zoey as a old friend, and nothing more, but didn't explain himself as clear as he could to Sage at his house. He's resolved of Zoey's issues, and is quite confident in his feelings for Sage. He would do this with any girl, and giving Zoey more attention in the narrative is foolish unless the narratives gives us a reason to. So far, there is none.
Sage is no longer scared. Her boyfriend is still hers, and he's not fooling around. Crisis averted. There's no need to tell her about Zoey's confession because there's no problem for MC or Sage.