Well if we're having degenerate debates... I'd argue there was indeed NTR in this story. Luca and Yuuta spent a bunch of time together, relied on each other, got close, were attracted to each other both physically and romantically, and then princess bitch shows up and just took him away from her. Luca was the one NTR'd by the princess, and Yuuta was the one stolen.
This is cheating, sure, but not all cheating is NTR and not all NTR is cheating. I have much more problem with NTR than just cheating; this is... Netorimodosare? taking back a stolen lover from the NTRing asshole. I'm all for it, go Luca!
Especially since there's still no love, connection, or even physical intimacy past the one time between princess and Yuuta. Marriage vows include "to have and to hold"; that means physical intimacy, including sex. Those vows among others clearly aren't being upheld. The marriage is a sham to begin with.