Kujo is a beta and Yao is his rival in the track club as well as an alpha. Everyone tells him not to bother since alphas are going to win by default, but Kujo is determined to try fifty times as hard. But when Kujo suddenly transitions to an omega, only Yao can save him from being raped by every boy on campus by sleeping with him. Yao continues to help Kujo keep his heats under control after that and eventually confesses, but just when it seems they've found love, it turns out that Kujo has already met his fated pair at their school, and the time of his first heat is fast approaching. This is a bizarre world where not pairing with your fated pair can kill you, and that took me out of it a bit, but other than that it's a solid story.
Trigger warning: attempted rape.
Bokutachi wa Unmei ni Kirawareteiru