I kinda sympathize with MC. When you're connecting with someone you can start to think they might be feeling similar to you maybe... You don't really think they're actively sleeping with other people. He probably read into the ML's actions and thought maybe they were connecting, starting to feel some type of way together. It's normal to get excited and maybe expect things when you start liking someone. That doesn't mean you're entitled to it though, as you aren't actually going out.
I understand that but I also understand Kashiwagi’s flat out rejection of any kind of budding crush. He drew his line right away. As someone that has been in his shoes (though in my case it’s because I’m ace), it feels really annoying for someone else to onesidedly get caught up in their feelings. Because while they’re getting excited and expecting things, I’m also excited about having a friend and then getting made the bad guy just because I didn’t catch feelings too.
Ahahaha that was actually kind of funny because Watase was so quick to not get upset knowing that he at least had a relationship with Kashiwagi that went past just being sex friends only to have that very same thought shattered within seconds. I know this comic will give Watase a little period of sulking before either he gets over it or it miraculously makes Kashiwagi miss him, but it’s funny to see characters get mad because their crush is living their life. He made it very clear and Watase is still allowing himself to get carried away.