he gets a do over, finds out his sect is a sham and still refuses his "First Wife" and her sect that is better for his pure body type? Why?

Not really "better". Yin and yang are opposites of a coin, they balance each other and are more compatible despite being "opposed". A sword is sharpened by conflicting with stone. Though yang girl and yang guy would be more compatible for elements, they would only be able to grow to a level that is allowed by the other. Frost girl's extreme yin is on par with mc's pure yang. Their conflicting elements can push each other further to form a more proper "balance by opposition", just like a grindstone sharpening a blade. Balance is represented by perfect opposition since the beginning of recorded Chinese culture.
Which is funny, considering a lot of Chinese culture has been avoiding opposition and change for decades.
I don't like how she is suddenly thrown into the story despite not having a single instance of screen time previously despite him talking about his past life many times(at least, not that I can remember). Maybe it was mentioned in the novel, but I don't recall her being spoken of here before.