It's a complicated topic, especially when it involves the scenario in the game. Simply cheating and saying you love your SO is outright betrayal.
But in this game, from Clara's pov, she knows she is betraying MC's faith but she isn't cheating for herself. She believes she is doing it for MC's sake and in her heart, she believes she still loves him. So I am telling it from her perspective. She is ready to do anything for MC.
Leon also said this is one of the reasons he isn't committed because such agents don't have to feel any guilt when doing things like this. He has a point there.
I am just going to leave this at that since each would have their own interpretation of faithfulness and love.
And here is the most wrong word you use, BELIEVES, in a relationship there is no belief, but being or not being in love, which are totally different things, a well-trained agent, both woman and man, knows that the risk of going to bed with the enemy to find out things is part of the package, so regardless MC should also know it in principle, if we talk about the correctness of your work towards us players, just as Clara knows it, in fact she knows it much more than Mc, being a born spy, therefore already in the preface speech, the couple is supposed to know what the mission entails, not just killing, Clara would not then be a spy, but just a killer from the agency, returning to the word BELIEVES, she puts it as before what his promotion, making people believe and believe that he is doing it for MC, all false, he can easily refuse as MC wanted, also because it was an option given, but she is aiming for something completely different, therefore another flaw in the story, not to mention that there they are long internal monologues, but choices for us players, I counted 5 in the whole game, mostly useless. and I say it calmly, NTRs are excuses, there are a myriad of NTRs that are appreciated for how the player can choose, true, that if you implement it to a small extent you know what happens, but here it is a track that is already well defined.