He had a condom -_-
but that other guy is obviously still wearing pants?????
The other omega still had his pants on in two of the pics I saw and from what I could see Dojin was to busy thinking about the Hyesungs expression as well, I can't see how they did it
Look at Alpha-kun's appearance after his ecchi with the other omega. The scene where he barges in the mc omega's room. This does insinuate a lot. So i'm going to agree ewww, Alpha was with one omega did not like him then practically forces himself on another right after?! Sorry but personally I don't find that ok.
I think it was just a lot of heavy petting/foreplay leading up to sex but Dojin was getting turned on by his thoughts of Hyesung and getting disgusted that the Omega in front of him was some other Omega instead.
Either way I think he acted really horrible but if he actually did have sex with that other Omega (and either way, Hyesung may think he did) I think it makes it even WORSE, if that’s possible.

Just. One. Word. EWWWWWWWWW!!
How dare he just put his dirty thing that moments ago was inside that random omega, inside my little baby (╯°Д °)╯╧╧