See, you're looking at it through a really cynical lens, like there's no point in telling Sage anything she deserves to know because "oh they'll probably break up after she graduates".
Like this, you make it sounds like MC should just fuck around while Sage is at work because there's no chance he'd get caught with her away, or he should dump Sage so he won't be starved of pussy for long stretches of time. Long distance relationships are hard, I won't deny that. But A) it might not be long distance, B) MC already knows what it was like and try to avoid the same pitfalls, C) MC and Sage are on the same page, unlike MC and Zoey a year ago. They want this to work, a family can be in the cards for them, but weighing the benefits of telling the truth about an ex wanting to get back together versus expecting the relationship to fail from the start like some business deal is not the way to go about it.
Nowhere did I say he was going to cheat on Sage. Maybe that can be inferred from what I typed. MC is 19 years old. I'm overlaying my thoughts based on real life. My only frame of reference. The reason it's rare for a freshman to date a senior is because of this exact situation.
Just because he doesn't cheat doesn't mean there's not a lot of pratfalls with a relationship like this. Overcoming the distance is one if it exists. Which it almost certainly will if he continues to live on campus. Also the time spent with each other is going to significantly decrease.
MC is a leader in a fraternity. Which has obligations of it's own. Sage could definitely attend the parties the DIKs throw. But anyone who has a job knows how difficult this will be to do & still do your job efficiently.
My point is there's no reason to cause Sage harm if there's no reason too. Even if they are going to have a perfect relationship, MC is going to be attending B&R with Zoey for 3 years at least. Sage has shown herself to have emotional issues & jealousy is one of them. With everything I've prefaced, how is Sage going to feel about MC attending parties with Zoey?
In your mind, MC can be completely okay with Zoey flirting & screwing other guys. But the reality is, he is not going to be okay with that at all. Maybe he will be.
But this confession & subsequent closeness is going to be on Sage's mind the entirety of MC's time at B&R when she's gone. He turned Zoey down. Confessed love for Sage. Bringing in conflict with his friend does him, Sage or Zoey no favors. My thoughts is just to wait to see how Zoey operates after being rejected. If she acts like Josy, by all means cut her off. But I believe patience is the right choice here.