What to say to students when they don’t want to write the argument for the side they disagree with?

Maybe this will work.

Sun Tzu, the Art of War:
It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles; if you do not know your enemies but do know yourself, you will win one and lose one; if you do not know your enemies nor yourself, you will be imperiled in every single battle.