First off, I'd like to point out that there is no such thing as "transgender marriage". Marriage has no gender. There are marriages, however, in which one or more of the participants is transgender.
Because transpeople are human beings, we have the inalienable right to get married. Inalienable...as in, you cannot take it away from us.
What people wish to do in lieu of being able to keep us from marrying is tell us who we can and cannot marry.