The princess.. I have no sympathy whatsoever for her, but its almost as she want the Hero to cheat on her by blueballing the Hero after the son is born. From what I catch with the story so far, it seems that the princess was (probably) okay with "fun night time" before. Plus that her faceshot looking at the MC potrait just seems a tad evil, as if shes plotting something.
If the princess is actually planning something, it'd be like this probably:
No siblings = no rivalry for throne + easier to control. MC cheating = can be outed as king candidate or even better for the princess for the MC to drop out and go out willingly. The princess can either rule the country alone or choose another "husband" for being a puppet king, with her at the background.
But if she's not, shes just being a truly inconsiderate person to not letting her own husband to do it even once in the span of 10 goddamn years. I mean contraceptives does exist in this world if she does not want another child or something.
Hope the MC can led a happy life with the warrior this time and not fell to the same rabbit hole he got it twice already.