
I think he already tried pretending he liked him once, in the past life.
There was this flashback triggered when the old chief eunuch told the mc to just "bear with it" for one night (as in go along with it, maybe even pretend to be fine with it), and the mc remembered he had already tried that once in his pastlife, which had not only resulted in the past emperor deciding to grant him a newly-created royal consort/concubine title, but also in the emperor deciding to kill all the other royal constorts that had made attempts on the mc's life before... So I think that's why he no longer wants to consider that option.

The thing is, it's not the consort title thing that the uke is afraid of, but the potential killing of the other consorts once the emperor decides that he "no longer needs them" once he believes he has gotten the mc's "obedience"... The mc thinks that it was those women's deaths that brought on the karma that made the two of them (the emperor and himself) get punished with trepeating the same kind of mess in yet another lifetime... so obviously he doesn't want to "cause" any more deaths in the same way now, lest they both end up with karma again and get forced to reenact this for who knows how many life cycles...
I have so many questions like for example what did the previous incarnation of the current king do when the uke killed himself. and the seme is an asshole buy at the same treats the uke really nice and also is obsessed I can understand why the uke would hate it and it seems the uke also hated that because of him people died, but people kept dying when he kept rejecting the seme and wouldn't it be like smarywe to pretend to love him and then kill him but then again, if it had gone for too long it might seem impossible idk I'm just rambling.