Okay so let's gather it all up together
>is married to the princess out of political necessity
>the princess is the highest power after the king so he can't just get out of it
>his stats are so high that he literally cannot have sex with the princess without causing her damage
>due to the inability they have to be an intimate couple the princess has grown cold and no longer sees him as a man because of his c*ck of literal steel
>the only woman he could ever love besides the princess in the world is a widow thus completely open and always loved him back
Can this even be labeled as an affair? this just seems fair to me (pun intended). Unless the author pulls up a 10 chapter story arc about the princess and how sad she is and shows her doing a bunch of training to take the D I have no feelings of sadness at all for anyone. Hell even if the princess was cheating on him I wouldn't feel bad about it, as long as the child is actually his her cheating on him would just be as justified.
Of course if she's been cheating on him all along and didn't let him know he had full open status on the down low then that's a problem but if the author has her find out about this and then cheats on him in retaliation I couldn't even be able to take it seriously, like, yeah, screw the guy who doesn't give you literal damage princess, do it while hero does the warrior woman.