After spending more than 2 years with the same person, you develop a relationship with them, whether you feel something or not.
Sage has status and influence, plus she's also the girl frat's pres, even adds more status. The leader of the Alphas could only go for someone like her, hehe.
You can see he doesn't want to hurt her when he dumps her in a dumb way. He waited until Sage developed feelings for someone else, so when she gets dumped, she'll have someone take care of her.
Also, why would even Sage ditch MC after he tells her about Chad's situation? Firstly, it's not his business so he shouldn't poke his nose where it doesn't belong. This can have huge consequences for 2 other people.
Secondly, Sage has her suspicions already about it. She said something like "You have a hot girl like me in front of you but you kiss her forehead? Are you gay?" to Chad. So, if she discovers his situation, or if MC tells her, she can add 2 to 2 and knows it's the truth.
BUT, I still wonder why Melanie was afraid of Sage. Apparently he's gay, so why be afraid?