So, thats exactly what I think.
Yeah, but didn't you know that even if it's the same body, and they are there to protect the main personality that they are different people? Did you know that people with multiple personality disorders can even have different allergies with different personaillities? Don't get me wrong I'm torn with this story about loving all the personalities and just the one. I do like how it turned out though.
This story actually deals with that. The main character has split personalities; his "main" personality and two "others." Each one of them has their own separate lover, and one of the personalities is even a seme. http://www.mangago.zone/read-manga/nounai_ren_ai_no_susume/
Ok, to all those who think the uke should love all the personalities because it's 'sad' that he only loves 'a part' of his boyfriend, here's the thing; SPLIT PERSONALITY = ABSOLUTELY DIFFERENT PEOPLE. But let's not worry about realism, because this is yaoi. So let's look at the story itself. From what I got by reading it, even the personalities in the manga treat it as such, with Yoichi saying that he won't give up and stuff. Moreover, I'm pretty sure that Yoji and Yoko don't see the uke as a love interest at all! So effectively, the uke is being faithful to Youhei by loving only him...