I think he can have sex since he had sex with Yeonwoo in the chapter without knowing he was the one who marked him, but he can't have children with anyone other than the omega that marked him. That's why he wanted to kill the omega that marked him, only that person (Yeonwoo) could give him children until they die.
Being marked by an Omega also means he can't smell other Omega pheromones, in addition to the baby thing
Ooooooh did you guess that from what Yeonwoo said after drugging him???
Also, can Keith still have babies with Beta women?
The marking system.is confusing. If Keith cant mate with other omegas because of the mark, then what was he expecting to do when he started to assault the MC (right before the MC bite his arm)
Like...wtf....