Or he honestly doesn't remember him, but he hasn't had a lover before (before he was told he had a lover [the lie])
I think there's several indications that Ui does remember but chooses to pretend not to. For the same reason he never got in touch after high school with Natsuno despite having been close friends but did stay in touch with some of the other classmates. Once Ui had made his confession he could no longer accept their friendship status. It was easier not to meet or to meet as strangers (I suppose to help keep the distance rather than having to be all pally).
I was just giving another point of view, but I believe you are right
While I'm pretty sure Ui did suffer from some memory loss, I'm also sure he lied about not remembering Natsuno. Then Natsuno in turn lies about their past and claims to be an x-lover. Now they are both going to have to pay for those lies. While Natsuno has to pretend to be a gay ex, Ui has to pretend not to remember him at all, the guy who was the love of his youth and probably still is. They have trapped themselves rather well.