Princess totally did that on purpose. She's paying him back for all the teasing he did to her.
He let the commander go but also let her kill the prisoners. He could've at least have fun with the dark elf, but no... Had to get all the blood up to his upper head instead of the lower one. He loves mind manipulation so much that he forgets about carnal pleasure.
also he auto imposes dumb things, like he hasn't touch her disciple in a while (if even since he brainwashed her), and pretty much with the commander, because when she killed all her comrades there wasn't soldiers there, or when he was torture/pleasuring her... still, he still accomplished what he wanted, pretty much forgot his carnal pleasure this time as you said.
I expected more build up for this girl, now shes kinda "just happened to be there".
I also didn't expect MC to be so "I'm a man controlled by my boner".
It's not like he was on an island of only males and just came back, any restriction was purely self imposed.
author forgot that he was doing sth with her, remember that she started to touch herself because he was taunting her with all the stuff he was doing, so this moment was totally expected (weird that she was doing it when he was getting awarded in the castle, why wasn't she there??), the build up was already done.