http://www.mangago.zone/read-manga/love_shuttle/uu/ibx_chapter-32/pg-25/ read this. so that u will be enlightened.
It was because he consumed a heat suppressor. The Alpha's rut was supposed to activate an Omega's heat, effectively numbing any pain but since Doyun underestimated how animalistic an Alpha in rut can get, he consumed his heat suppressor and expected a normal slightly rougher sex and boy was he wrong.
LOL why are you trying to argue for everything? in your own words, if you don't agree with me dislike my comment and kindly move on hahaha but since I have unkindly stayed behind to respond... the knot and fetish isn't even part of my original comment... my concern was with how everyone seems to find every single thing as "sexy", and although that's a large part of the attraction to this story, anything else gets overlooked or ignored. Fucking through the pain is hardly sexy in my book.

I expected him to thoroughly enjoy the sex... Instead we got a mindless animal that made him cry of pain and pass out with his ass numb.. And still he kept going.
.... And still there are fangirls calling this sexy. Girls I don't believe that was the intent of this chapter.