How is anything of this NTR if she divorced first before running back to MC's side lol
If anything MC was being dense on purpose to deny what he felt because he thinks he's a useless man in every way. I'm guessing he thought he could only give her a very miserable life and that's why he let her go in the past so she could marry the king. And given that the previous dialogue implied he's sterile in addition to being non-magical I get why he would feel that way.