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To say ethics are arbitrary is to say that there is no reasoning behind them, which is basically the opposite of what ethics truly are.

Ethics are commonly a "system of moral principles", said system and principles being the reasoning behind ethics.

I'm assuming what you are trying to say is that "ethics are not absolute", which has strong arguments; however, I think the critique of the use of "unethical" is partially in agreement with my assumption of your first point ("ethics are not absolute"). Assuming ethics are not absolute, then labeling something as unethical in a blanket statement would not always hold true.

I would suggest simply prefacing the statement with "I/many believe" would sit more comfortably.