It's possible, because the tax code is complex, but married-filing-jointly has a lower tax rate.

For example, let's assume you both have 40k taxable income:

Single, taxable income 40k: tax is $5,935

Married, taxable (combined) income 80k: tax is $11,864

Double the tax on "single", because you would file two returns, and you get $11,870. So you actually saved $6!

Taken directly from the tax table: http://www.irs.gov/pub/irs-pdf/i1040tt.pdf

Yeah, using these figures our married tax liability still ends up being more. I suspect because our incomes are very similar and there is a steep hike (5%) between tax bracket we would be in individually and the one that applies to us if we got married.

Yeah, there are some nasty non-linearities in the tax rate. (Although you do know it is incremental tax rate, right?) My point was mostly that "in general" it's generally a wash.