I can't really sugar-coat the Sage drama. The mc was happy to fuck her while she was angry that her boyfriend was cheating on her, and the mc even kept hinting that she should dump Chad and move on. Then he goes all rebound snowflake when she and Chad are over.
The only thing that gives the rebound reaction any traction is that it wasn't Sage who did the dumping; in the end Chad dumped her and she took it hard, and then she wanted to jump into the mc's arms. This wasn't the scenario the mc was looking for.
Maybe I kinda get it even...
If Sage had dumped Chad for the mc, that would be a positive. But Sage only got interested in a relationship with the mc after
she got dumped. Rebound alarm bells!
Looking at that scene, right before they start sparring, the mc says to Sage:
mc: "I understand what you went through...or what you're going through."
mc: "Break-ups are tough."
So when Sage starts kissing him and looking to get physical after the sparring session, the mc is like, "Hang on, you probably need to take step back, I'm not just here as second best after Chad dumps you..."
So yeah, it wasn't what the reader wanted right then and there, but was it ridiculous that the mc behaved that way? Not really...
Maybe...