I do see 218 raised as a point against the series a lot, and I can't blame anyone, but I can offer a bit of a different perspective.
I have generalized anxiety disorder, and what happens to him in that chapter is a negativity spiral. Your brain latches onto one bad thing and then just keeps going with it, deeper and deeper into some delusion which only makes things worse.
When that's happening your brain and body start doing some REALLY weird stuff which most often serves to further amplify the spiral you're in. I've had a VERY similar thing happen when a friend I had feelings for started dating someone else.
It's unpleasant to watch, but it is somewhat realistic.
I get the gist of what you are talking about, as I have had this happen to me (negativity spiral). But let me offer this point from my perspective:
It would be more realistic, if say a negativity spiral were happening in this case where, he is imagining himself chasing after her only for an argument to ensue and the argument gets so bad, things are said that cannot be taken back and the bridges are burned completely. An possible relationship is now impossible in this imagined scenario.
But with this, we are just seeing her in the arms of another guy(s) and being railed by him. From my perspective, the mere fact he cuts to this, makes it looks like her being sexually gratified by another man is completely unacceptable to him and he immediately despairs. It's not the complete and utter breakdown of their relationship. It's this. As if the act of sex with her is the goal being snatched away from him entirely, not the building of the foundation of their relationship. Which, as stated before, makes our MC out to be very very shallow.
Now this scene would be fine if they were both married and his anxiety spiral was him freaking out over the mere thought of her cheating on him, having him jump to this as a possible mental scenario. However, we are not there at that particular situation.
Sorry for the long-ish post. Totally get with what you were saying. Even if I do disagree slightly, I enjoy the conversation