In this specific universe, it is a way for Omegas to make sure their mates/significant others can't run off and impregnate other Omegas, and can't even smell the pheromones of other Omegas. And the only way to get rid of an Omega's mark is for that Omega to die. Alphas can still mark Omegas in this universe, but those marks are only temporary. They can be overridden by other Alphas, or just cease to be if the Omega so chooses.
Considering that in most omegaverse stories, the Alphas are the only ones doing the marking, markings are usually permanent, even if the Alpha dies, and it stops the Omega from having sex with anyone else without getting sick, it is a nice shift in the power dynamic.
Yeonwoo is happy because right now he wants to see Keith suffer. What better way to make Keith suffer for raping him, than to tell him that he is the one that marked Keith, that he is getting rid of Keith's baby and that he plans to live a long life as far away from Keith as possible. It is basically letting Keith know that he can never have any kids as long as Yeonwoo is alive.
It also adds that Keith can't smell other Omegas now, so his chance of ever having another Omega would be greatly reduced. Basically, at this moment in time, he has essentially crippled Keith.
I see, thank you for your detailed explanation. Actually, I wasn't referring to his reaction in giving keith the marking. It's just, I was kinda confused as to why he even use that question to remind himself that keith is still an asshole when the whole point of marking (as I thought before just based on other stories), is to cut off any chance to connect with other omegas/not let them have sex with others. Turns out it's not completely impossible (ok I now rmbr there were others like that too).
Still, he's setting the bar too low lmao just cuz keith didn't took the smallest chance to have sex, he thought he kinda improved?? But I guess him being in a rut adds in to it. Kinda funny he noticed that and not remember that he basically set him up with that bounty, disregarding his safety.
what was the point of marking again? and why is he so happy that the cheater didn't have sex with anyone during his rut when he already marked him?? i know thats already a milestone for that guy but why is this omega so happy?