its been awhile since i read the novel but no, he did not have an affair of his own volition with harold. iirc he was drugged/in heat (i forget which) and thought harolds dominant alpha pheromones were winstons, and i dont remember if they went all the way, but regardless it was unconsensual on yujins part bc he didnt know who he was with. it didnt help that harold was essentially grooming yujin, and winstons family believed yujin to be a homewrecking slut (they purposefully fed winston lies about yujin, and walking in on harold and yujin nailed the coffin shut)
Damn
Someone tell me, did the omega really had an affair with the "Father" or he only looked up to him as a father figure but everyone misunderstood or did the "father" do something to him