Remember, if you are married on December 31, you are married for the entire year. If you are thinking of getting married in 2019, you might want to rethink that plan for the same reasons that apply in a divorce (as described above). The IRS could make big savings available to you if you are married on December 31, 2018.
Again, you have to run the numbers in your tax return both ways to know the tax benefits and detriments for your particular case. A quick trip to the courthouse may save you thousands.