tl;dr -- The tax disadvantages of marrying are not always minor. Marrying cost us ~ $20,000/year in additional taxes.
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One particular disadvantage of marriage from a tax standpoint we discovered is the phaseout of certain deductions at a higher "Modified Adjusted Gross Income" (MAGI). We live in a very high cost of living area, and we both own rental property in our area with extremely high real estate prices. Although I've owned other rental properties, we didn't set out to be real estate investors in this case -- we had to convert our individual homes to rentals in order to combine our households.
But many of the deductions on writing off real estate expenses start phasing out at $100k MAGI and are eliminated entirely at $150k MAGI. Those limits have not been updated since the freakin' 1980's. And there is no consideration of the higher cost of living or value of the property, just a straight income limit no matter where you live. Which is pretty stupid, IMO, since a married couple in rural Kansas with an income of $150k and $900k in rental property would be considered exceedingly wealthy. But in SF or NYC, that would be a modest income with perhaps a single, 1-2 BR rental condo.
With our combined incomes, the year we got married the tax hit was gigantic -- to the tune of $20,000. We went from a $10,000 combined refund while single, to owing over $10,000 once married :-(
I went through it painstakingly, the difference was all attributed to exceeding the MAGI thresholds with our combined incomes. Being unmarried, we would have saved $20,000 (choosing a filing status of "married filing separately" doesn't work -- the IRS considers your combined incomes for MAGI purposes as long as you're legally married, filing status does not matter).