What I never asked.
'There was a time where I latched onto other people like a parasite, and I clung to them because I didn't have anything else. I was empty. I let them use me because I didn't have anything to lose and wanted to feel something.'
This was how Sumire described her relationship with Nagawa Sano to him. Yuhi clearly remembered the look on her face when she said those things and her pained voice. She looked so lost and fed up with everything.
He has no right to complain if she slept with other people. After all, didn't he do the same? To get rid of the loneliness and pain, he did careless things to cope with the pain of not having her.
If Sumire felt the need to do that despite being with Mamoru, then perhaps there is more to that relationship than he thought. She is not like those women who would cheat on their partner because they felt unsatisfied; there must be something he is missing.
What kind of relationship did Sumire and Mamoru have?