Well, arguments of the form 'if we allow X then we must also allow Y' is a fallacy in the strict sense that it does not follow with logical necessity when X and

Well, arguments of the form 'if we allow X then we must also allow Y' is a fallacy in the strict sense that it does not follow with logical necessity when X and Y are different things. While if you accept the premises that say X leads to Y and Y leads to Z, then it does follow logically that X leads to Z. The word 'fallacy' is often used as synonym for 'wrong' but that is misleading.