Chad has never been in to Sage. In the flashback at the beginning of episode 5 he was already blowing her off (Sage was just too love struck or something to see it - look at the poor innocent face, idolizing that beefcake...)
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And look at Chad's face. He is perpetually tormented. He never looks happy.
During the fight they have just before the MC comes around to teach Sage some more guitar, Sage complains that it's about time Chad started getting more physical with her, right after he gives her a peck on the forehead.
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She's not saying, "how come we're not physical anymore" (like what might happen when you start to fall out of love with someone), she's saying it's about time they "start" getting more physical. That means, in the 3 years(?) of their relationship, how physical has it even got? Certainly sounds like there hasn't been any sex.
Also, earlier in their argument she accuses him of only paying attention to her in front of other people:
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These are all major flags (not wild speculation) that he's batting for the other team. His relationship with her is all for show (to hide his true sexual orientation), and once they are in private she's just his friend, and he doesn't deserve her.
Yes the perfumed undergarments do pose a difficulty in assisting to explain all this, but I can't fucking answer everything!