Even though the 1996 of the DOMA – Defense of Marriage Act may be going away which is a great thing, at least for now, lesbian and gay couples may not use either of the married filing statuses. (married filing a joint return or married filing a separate return)
So unless you are able to file as a heterosexual couple or married filing a separate return, you must use the single filing status.
Your filing status affects both the size of your deduction which will be lower as a single filer versus a married filer, and also tax rates are lower for married couples filing joint returns versus single filers. Also numerous other tax benefits (deductions and credits) are not offered single filers.