Responses
You know that actually Shiratori manipulated Sumi?
At the beginning he did was ok with any alpha, but then he just stop saying he wants an alpha and change to Shiratori and reminding him that he needs an alpha (and he feeling that is a burden to Utou), even in the night of the bond everything was prepared to make Sumi beg him to be paired with him
I never like the uke in the first place and I felt like he got what he wanted. he wanted to escape his heat so badly that he any aphla will do and happen to meet an aphla that hated the semi so there's no reason to pity him. He got what he wanted, he deserved it there's no reason to regret it anymore.
I admit that it's sad that the aphla that happen to be nice to him that really using him in but what if shiratori had actually liked the uke for real and his heart was with somebody else that he's just using shiratori. I don't see anything wrong in using uke since he is doing the uke a favour and they are bonded mates now so.......
This is all my opinion they both entered the relationship in order to use each other for their own personal gain so if one if one actually uses the other it doesn't matter because both of them had the reason for being in that relationship.