I... don't get your question. He just... is? He could also be bisexual. Just like I'm just straight. Like, idk why. I just like dick and only dick. However, he's obviously not out yet (although, it's unclear whether or not his frat brothers know, but I kind of think they might... or at least one or two do... Dawes is the one I'm thinking of), so the potential reasoning behind him being with Sage really depends on what Chad's sexual orientation actually is.
If he's bi, then him dating Sage is aligned with his sexual orientation; if he's only interested in men, then the most likely reason he'd dated Sage so long would be that he's not comfortable being out yet for whatever reason(s) and he stayed with Sage because she was pretty much willing to tolerate anything (as we've seen). The other possibility is that he wasn't able to explore his sexuality when he was younger and only realized he liked men when he got to college.
There are plenty of potential reasons for why Chad isn't openly gay/bi, and I think saying he "suddenly turn(ed) gay" is just... not what happened, and viewing it that way completely disregards the fact that being openly gay is still not necessarily acceptable to everyone (which is trash but like... Am I surprised? No.) so some people might not be comfortable/confident enough to come out -- which is actually depicted in the game with the Maya-Josy situation. And also, it's weird to suggest that pills turned him gay in a game that has no weird science shit going on... so, no. I'm pretty sure it's not the pills.