To my understanding, a person is a person under the law. Once your gender is officially changed, according to the state, you are legally that gender and they can't treat you any differently. That's the one advantage to the law being oblivious to transgender issues, for the most part.
Now if your legal gender makes your relationship a homosexual one, then the states may weigh in (for now, but that's changing too). For the most part, legally, it's a gender/sexuality difference. If you are a female with a male partner, or a male or a female without a partner, adoption is an option (although individual judges/social workers can cause issues).