i think he didnt mark him, because it's a life time desition, and tae knows how is doyun with such a serius thing, my guessing he wants to wait for doyun to make the choise, as per the reason he knoted, well alphas dont think too straight when one of them are in heat or rut, they can make some desition but in pleasure is harder
I'm thinking along the lines that due to Doyun's heat cycle being irregular, this may be a factor to his ability to get pregnant. Same thing happens to women who have irregular periods. They have a more difficult time getting pregnant. So it is really a hit or miss situation and Doyun may not even be pregnant yet. Otherwise, it seems they would already have a litter of pups as knotting has occurred before and...nothing.
Usually... in abo yaoi stories... the alpha does the marking and knotting at the same time while making love with their omega, and then boom, the omega’s pregnant the next day. But here, Taehan knotted Doyun first... and let him walk free in public without marks, which really makes Doyun a prey to alphas cuz he will spread pheromones everywhere. Doyun has Taehan but he is not claimed. No marks. He must be pregnant too, idk. I just know that they should do the marking. Doyun needs to be claimed. It’s the only way to make sure that no alphas will make a move on him. It’s like, the proof that he is mated already. So yeah. DO IT BOYS!!