Some Interesting Points to consider (These are not criticisms. Just observations. That's why, they are too much long):
In chapter 3, Kat is the one who first proposes to spice up the marriage. Then, you will be given the option to react on that. So, the "desire" of sleeping with other guys is "inbuilt" in kat's character traits. It's surprising & out of the line for kat's character to warn MC with words "No third parties" immediately after MC select non-spicing option (vanilla route).
It will be coherent for her character's model if she say something like "okay, no spicing ah? No problem, I will wait for your interesting idea then". I think the warning words "No third parties" may be just created to appeal the Non-NTR fans with no logic.
2nd point:
When Kat's Boss came to meet MC for a detective case, MC mocked kat's boss by calling him a "cuck". Just because Boss's wife slept with another man, that's not NTR or cuckold. That's plain cheating. Boss don't even know about that cheating. So, No jealousy involved.
You know what is "real cucking?", when kat's boss is fucking kat like a fuck doll, and she is submitting to him to even "never tried" kinks like rimjobs, while MC is hiding behind the corner, watching all that with kat's agreement & jerking to it, that's called "real cuckold". Yes, No humiliation is involved for MC. But humiliation is not important for one to be a cuckold. You can enjoy cuckolding without humiliation too. For example, The game called "The Adventurous Couple". So, yeah, the real winner is kat's boss. He may got cheated. But, he get to fuck a submissive sexy fuck doll like kat, who is so much better than Boss's wife.
And talking about dom/sub, have you observed how kat behaves around trevor & boss? Trevor spanking her, not leaving her even after she says to stop, calling words like "cheating slut" etc. Kat even tries to behave as a "sub" towards MC in non-sharing route too, but MC is less dominative than Trevor & Boss. Kat is the who proposed rimjob in chapter 11, but not the MC. and MC never hard spanked kat like "trevor" did. There comes a option in one chapter we have to select "make kat your lady" or "make kat your slave", if you tried both options, MC got success in making kat his lady, but failed in making kat as slave. Kat even mocked at him by laughing that he failed. But, surprisingly MC is much dominative towards all other ladies except for his wife kat. So, those who are trying to make kat as a sub in bed in non-sharing route, even though kat has fuck-doll traits in her character, tend to get disappointed, because of MC's incoherent behavior towards his wife even though he have dom traits in his character.