He does truly love her. Just not in a way you can understand.
The problem that at least half the readers seem to have is that Kirishima can, in fact, see the difference between love and sex. It's actually because he compartmentalizes the two that it's so hard for some readers to wrap their minds around the fact that he sees love and sex as two separate things. As far as he's concerned, to love her has nothing to do with taking care of his basic sexual needs with any other willing woman.
So is he a conventional ML? Heck no. But I personally signed on for this because I knew I’d get to see blood and some F’d up characters whose sense of morality had nothing to do with me and now I’m here to stay. I also came to see her fall for her yakuza fiancé and I ain’t complaining; I got exactly what I signed up for. He’s not some Prince Charming.

Y’all I’m sorry but that ml just ain’t it for me but I’m simping hard for our mc and she deserves better (mc can step on me akbsjsbdksnk)