A progressive income tax is constitutional because the 16th Amendment explicitly authorizes it.
Wrong, the 16th amendment authorizes the income tax, but the progressive rates would violate equal protection under the law. Everyone should pay the same rate.
Not that I agree that the Constitution says that because the law is based on income so if you make $X, your tax is X% and everyone abaides by that law. Just like the marriage law is man and woman. Even gays have the right to marry someone of the opposite sex.