Under the current system, all income is taxed the same way (W2 income, ignoring investment income, etc). The first $10k (or whatever) that we both make is taxed exactly the same.
The next X amount is taxed exactly the same for both of us. If that's all the money you make, that's all the taxes you pay. If you make Y on top of that, then Y gets taxed at another rate. If I also make Y, I also pay the rate on Y.
Thats fair.
Its not unfair that we tax the twenty thousandth dollar that you make at 15% and the three hundred and ninety thousandth dollar that you make at 38% because everyone is subject to the same schedule.