And
Wizard_Shiryuu's point was that because Riona is an official side girl of the MC this pretty much rules out her being Chad's side piece.
If Chad is cheating with Mel, then yes, that is a disappointment and a betrayal, but it's not nearly as bad as finding out that the person you liked and possibly even loved wasn't even interested in your gender, let alone you.
If we think about this in terms of "what's most devastating" (since we tend to expect DPC to go down these tyes of paths), Chad being gay would be more devastating than simply cheating with Mel. In the Mel scenario, it could be presumed that Chad did at one point like Sage, but then his eye wandered to Mel, and this is pretty standard hetero cheating which would hurt Sage, but it's something she can probably more easily get over. However, if he were gay the whole time, then this means he was never interested in Sage and was essentially using her as a beard, and that would hurt Sage a lot more.
We know from Ep 6 that Sage already feels like she's not good at processing and expressing her feelings well because she comes from a family that bottles things up and doesn't easily open up to each other. She seems to doubt herself already with regards to her feelings for Chad, so imagine what would happen if he were revealed to be gay and she couldn't tell. It would make her doubt her judgement of people, which could be compounded later if/when she finds out about what Quinn's up to, and this could cause her to doubt her feelings for the MC and keep him at arms length from this point on.
If DPC doesn't want to makes things easy for Sage and for a MC/Sage relationship, then Chad being gay would be one way to go about doing that.
So you weren't bothered by the fact that one of main characters, specifically a LI, had to die in a house fire? Even if you weren't interested in either, it's still a heavy blow for what is essentially a porn game.