I have been wondering if Chad isn't in the closet and having a down low relationship for a few episodes now. Just seems odd how close he's keeping this side person to the vest and we never see him chatting anyone up outside of his frat brothers. I mean, the phone call didn't give much away and men can wear panties too if they want to.
I was also wondering this when the envelopes were slipped under the door and then Chad came to fight MC.
Especially since there were two envelopes. One for Chad and one for Dawes. I'm pretty sure the pics Chad showed the rest of them were only about Arieth, which would likely be the contents of Dawes' envelope, meaning that Chad kept his envelope secret.... However, it could just as likely be that the contents in his envelope are pics of him and Melanie, although I'm not sure why he'd feel the need to hide that from the other jocks.
Either way, "surprised" or not by Sage saying something about it, he definitely cheated on her with someone.
P.S. I'm a woman - and cheating men honestly, truly believe they won't get caught and will deny that shit until there's no other option. Why? Because they're stupid, I guess.