Hmmm thats true, and I don't deny that the omega is an opportunist. But its a bit more complicated since this an omegaverse, especially one that isn't really taking anything seriously.
While you can wait out a rut, they are eased and end sooner when they sleep with someone. Thats why the omega took the opportunity. Most alpha's would usually be grateful to someone willing to ease their rut. Still not fully consensual but isn't like the most terrible thing in their world

how is mc better than the shitty fiance he's trying to break it off with?!? just because you're in an abusive relationship/arrangement, doesn't give you the excuse to manipulate and take advantage of other people. He orchestrated that whole encounter/rut just to trap the seme into marriage. I don't care about his reasons, that's disgusting!!