Yeah, that chapter ruins the entire series for me. Dark topics were handled especially well and awkward stuff was actually awkward, until suddenly, MC breaks all his principles.
Here is a question that will break any female that supports "open-marriages".
So what happens if the concubine gets pregnant first?
Think about it, the order of succession is firstborn. If you are just going to overlook the firstborn for the "plausible" kid with your "legal wife" then you are basically discriminating your own kids.