Okay, let's put it is a more clear perspective. There really is little or no advantage with the current tax policy.
From the tax tables:
A single person earning $20,000 AGI pays $2,586.
A married couple each earning $20,000 AGI and filing separately pays $2,586 or $5,172 total.
Filing jointly that same couple with a combined income of $40,000 AGI pays $5,169 so you can see there really isn't any inequity except that married couples filing jointly get advantage of a higher standard deduction under the Bush tax policy. They will lose that advantage in 2011 if Congress does not act to keep it and will be paying more.
Also if the head of household is the only one bringing in income of $40,000 AGI and the wife is home awaiting the birth of their first child, and they file as singles - she pays zero because she has no income. He pays $6,194. (she actually gets a check even though she didn't earn under bush's tax policies)
In my opinion as I said, there are far more advantages to encourage people to get married even if they get a tax break to encourage that. The reduction in child poverty, the better schools, the more stable, prosperous neighborhoods that result more than make up for any 'lost' revenues to the national treasury.
If i "cherry pick" an example I can show a greater bias than your example.
Look at $40,000/year + individuals and the inequities become very clear.
Here lets try 70,000/year:
A single person making $70,000/year is responsible for $13,694/year in federal income tax
A married couple making $70,000/year are responsible for $9,881/year.
Thats a difference of almost $4,000.
BUT, if that couple is each making $35,000 per year and file separately, they each pay $4944 or a combined total of $9888. If they marry and file a joint return, without any tax break they pay $4,000 more. However, the marriage tax break allows them to qualify for a larger standard deduction that saves them that $4,000. That is an incentive to marry.
It's all in what promotes the general welfare. If you want traditional families that produce improving stable neighborhoods, lowered crime rates, better schools, more beneficial environment for kids, less child poverty, more productivity and opportunity for all, it makes sense to promote the general welfare by granting everybody--not just a select few but everybody--an opportunity to take advantage of tax breaks for married couples.
If your only goal is to make sure nobody gets anything that you choose not to benefit from yourself, then you take away that tax break for married couples. But you also take away one of the incentives to marry which would likely produce unintended negative consequences.