Because the game strongly hints that he is.
As
RanchFTW just said, the game is careful to avoid revealing the gender of Chad's "mistress." The jocks have a homophobic culture so over-the-top it can easily be read as overcompensating. Sage says she's only good enough for Chad at parties or around friend - ie in public. In private, we see he treats Sage as one might treat a relative (affection, but no sexual interest). And Chad completely flies off the handle when he sees the blackmail pictures (which we do not!), implying the secret is pretty big; since Sage already knows Chad is cheating on her, it's unlikely to be JUST that.
All told, the evidence is clear. It could still be intentionally misleading, but it is definitely trying to make us wonder.