Except you did. You used that reasoning to support the claim that "taxes should be progressive."

"... Flat taxes fall most severely on those who are least able to shoulder the additional and unfair burden.
Taxes should be progressive to reflect the varying degree of benefit derived from said tax."

In any event, I tend to use philosophical terms. Any statement in favor of or against a point is an argument by definition. Even just laying out the reasoning involved in getting to a particular position is, by definition, an argument. It needn't involve any conflict between two positions.