@Sensiteeve27—
As I already said, there are many different codes of conduct. Simply insisting that the code that you embrace is
the code of ethics isn't a useful argument. Telling me that some action is unethical because it's unethical is a waste of electrons.
And if you're going actually to make a case for
some code, then you need properly to identify
which code. You cited
the bro code, but you want to dissociate yourself from its
central thesis.
Yui
hasn't gone behind anyone's back, and Chitose
isn't married, which is to say that she
is single. If you want a serious discussion, then your attempts to frame the situation need to conform to the story.
Yui has been honest both with Chitose and with Kinosuke. And if Chitose and Kinosuke are happily in love, then Yui's professions of love will not pull them apart. (In fact, they are, and his situation looks hopeless; but his actions don't look unethical.)