Tax system unfair to stay-at-home parents

I'll tell you just how unfair the tax system is to couples on low incomes who want to look after their children. Just look at this example. Why shouldn't the tax system help married couples where one gives up work to raise a family? They should be treated as a unit.

For easy reckoning I'm assuming the Tax allowance is £10k as promised for the next budget. So Man and woman (or civil partnership couples) earning £15k each have a joint income gross of £30k , but each has a £10k allowance so together
that can earn £20k and pay tax on the last £10k, net income £20 tax free.
Now if one chooses to stay at home and look after the children, they lose £10k of tax allowance so even if one is lucky to earn £30k they still only get £10k allowance so joint income is reduced, only £10k is tax free they have to pay tax on £20k.

So I ask is that fair on married couples sacrificing income to bring up children, leaving a job open for someone else but paying TWICE as much tax on the same income.? How is that fair.

Come on Cameron you're Married and a dad. If your income was only £30k what would you prefer?