Hey, glad you liked it!
As for the scene you're referring to, Born to Breed isn't really romantically focused like that, so I would kind of hesitate to call it NTR. There isn't cheating or anything like that involved. It's more like friends with benefits, but with more feeling.
So I guess it depends on how you define NTR, but I, as someone who's not really into NTR, wouldn't classify it as such.
Now, to actually answer your question. Born to Breed is a (mostly) linear story, and that scene is a part of the story and the natural development of the characters, so it isn't optional.
I hope that helps - sorry it got a bit long!