It's the easiest way to get him out of the picture. If he's straight, he's a threat, if he's secretly gay, he's not. I guess those panties could've been worn by an effeminate guy, right? He also, as Ranch pointed out, showed no interest in having sex with Sage when she offered, and clearly the natural assumption if a guy doesn't want to have sex with someone like Sage is that they must be gay.
At this stage, with what we know, either one could be possible; secretly gay or cheating with another girl. People latch on to the gay theory because it's both a convenient way to get rid of Chad, but also would be something different than a typical straight affair.