Before, sex reassignment surgery (SRS), or “bottom” surgery was technically required, though trans men were frequently able to get around this by submitting purposefully vague letters from their doctors about the medical treatments they had received. Now, SRS is no longer required. All you need is a letter from your doctor stating that you have received “appropriate clinical treatment for gender transition to the new gender.” Here is the official federal document that explains the new requirements. And here is the National Center for Transgender Equality’s explanation of the new procedure for changing your gender marker (scroll down to the Passports section). This will make international travel immeasurably safer and less anxiety-producing for trans people.
Since a U.S. passport is considered a highly official form of identification, having the correct gender marker on your passport may open the door to getting your gender marker changed in your other I.D. documents. Since changing your gender marker on your U.S. passport is now far easier than changing it on your either your birth certificate or your social security card, you may want to get a passport reflecting your correct sex even if you are not planning to travel abroad in the near future.
Take a look at my new page on trans legal, medical, and other resources.