Mitsu ni Kiba

In this world, where omegas are rare, there is a rumor about a problematic omega who is like a honey trap... None can escape his seduction. Omaki Kazua, who was waiting to get his secondary gender test results, comes across Kurogane (an alpha) and Nadame Sachiyuki (an omega) having sex at school. Sachiyuki approaches him, claiming that they are "Fated Mates," but Omaki and the others refuse to believe this. However, Sachiyuki, who turns out to be the infamous "Problematic Omega," refuses to back down.

(Source: MU)