Its not marking tho. It's just senting, like when you leave a hicky on your partner to imply to others that the person is in a relationship, scenting is just a more primal way of doing it. I do admit that Mathew should have told him though.
And where are you getting this information from? because none of these chapters have mentioned "scenting" as a thing.
I'm using the 'scenting' term because what he did is considered marking in this book but it's different bonding to your partner, just go to chapter 27. The author explains the way the dynamic works in this book.
So essentially what I'm saying is that he didn't mark him in the way you're thinking.
I'm respecting the fuck outta Jaeho's glaring right now. I'd be looking at Matthew like that too. Completely irresponsible of him to do that!!