If you're from a disadvantaged background, but put as much effort as a person from an advantaged background, then you would make the same after-tax income.I think this misrepresents the notion behind progressive taxation. I'm no expert, but I understand it like this: because of declining marginal utility, the same % of income may mean much more to the poor than to the rich. If my yearly income is 10k, then 10% of it might mean the difference between making rent or not. If I make 100k, 10% of it may mean forgoing some luxury, but needn't substantively impact my overall quality of life.
In that sense, the subjective impact of money is not fixed by its objective value; it's more important to those that have less of it. This is borne out by research. Money significantly impacts happiness until you make enough to basically get into the (lower) middle class, and then it has little impact.
So, if you buy the above intuition and want to use taxation to help maximize happiness (utilitarianism), then you would want some form of progressive taxation in order to alleviate the negative impact of poverty. This doesn't mean "making everyone equal."
To test our intuitions about this subject, I like philosopher John Rawls' conception of a "Veil of Ignorance." Rawls would ask: would you rather live in a society with, say, 1% billionaires and everyone else in poverty, or one more evenly distributed around a middle class -- without knowing in advance where you would end up in this society? The answer may well depend on one's level of risk tolerance, but I'd wager that most people wold opt for the latter.