Two things.
First, remember the scene in the Ep5 prologue? Sage totally had a crush on him, he was completely indifferent to her. So why did they wind up together?
The other thing is his reactions. When the jocks encounter the MC after the cafeteria fight Chad is tightly controlled, he's going to chastise the MC for embarrassing him in a public situation. Then, next thing you know he's punching Big A in the stomach. Chad totally looses his cool. Why? Gay slurs.
Then look at the attack later on. Chad later admits that he lost control, went too far. If Chad was just cheating with a girl behind Sage's back, it wouldn't be that big a thing to him. The Alpha's would be slapping him on the back. But anytime someone get's close to that subject he over react's.