I don't even see how the author can repair this honestly, because now that he's done this he's probably gonna end up:
A. acting like this never happened and never have it mentioned again in the story
B. making some sort of excuse like "oh it was in another dimension and the palace made me enjoy it"
or C. saying "fuck everything" and NTRing Kazuya with Kotetsu
I feel like the author might just play it safe and ignore it, but we'll see.
Man , you are seeing waaay too deep into it.
Really,
This manga is top-degen material and what kotetsu did with Sunao this chapter, was nothing that was not seen before, if you look at his Pixiv or twiter. If you consider this a signal for NTR i will tell you , you are way too sensitive. I dislike, even hate NTR. But this is not it.
Kotetsu its no villain, has been a companion of Sunao since forever and never could do anything of the sort, just as Sunao says "Its taking a once-in-a-lifetime chance".
Also, Kazuya has in the past accepted to do "spiritual intercourse" with men, didnt do it, but agreed to.
SO if anything, kotetsu would end up as one in the Harem, THAT would be soo funny.
Anyway, stop taking it seriously it was a bone for Kotetsu. You know? part of the good guys? not the mega-psi-rapist?