Take a break if you want to man, I'd understand it.
At the very least it's foreshadowed in Episode 1 so it's not like New Horizons to where it's "foreshadowed" that one of the characters is a "prostitute" about 5 to 10 minutes before it's confirmed true after you've already played hours of content.
Anyways, for my actual full scale opinion. I think it's fine, I despise ntr and that scene did nothing to me other than what I think it was supposed to do. Which was to make me understand why the MC is trying so hard to "be" someone else. So I'd label it as the "good" form (it actually hurts me to say "good" and ntr in the same sentence) of ntr, it's a skippable past event that already happened to the MC, the MC wasn't into it at all, nor did he actively encourage it, and it's used as a key facet of his character, his story, as well as the main story. Plus he didn't watch the "full" scene, the shock caused him to collapse to his knees and knock the door open.
It's a major plot point that his ex cheated on him, happens in real life too and while it's understandable that people don't want that kind of stuff to happen in VNs, it can still be used to tell interesting and entertaining stories like Echoes. The way I see it, the MC is using that experience to try and get his life together and improve, to try and be more active and less secluded.
My only potential problem with it is if there would be any forced ntr in the future, like no matter what choices you make the MC agrees to some sort of sharing or one of the girls cheats on him, THAT would make me upset and switch sides.
I say keep making the game, after all the scene IS labeled as a "side girl" which pretty much means its completely optional to get back together with her, and it's not even confirmed if it's even possible to do so. BUT, your health is the most important thing, so take a break. However long you need, I'll gladly wait. Take care man.