In the day when a man can become pregnant because he used to be a woman and was still a woman on the inside, this is not a moral issue.  It’s a practical one.

You have these different groups: male to female, female to male, post-op, pre-op, non-op, infants surgically assigned, transgender full-time, transgender part-time.  One partner or both.  In some states, your gender is what is on your birth certificate, and not everybody has access to one.  In others, it’s what’s on your driver’s license, and you can petition to be assigned male or female.

If you are transgender, and you fly from San Francisco to New York City, it’s quite possible your gender will legally change 8 times before you land, because states may not share your own state’s view of your Gender.  

What does this have to do with being Gay or Lesbian?  Marriage is defined by gender.  So if one or both partners are transgender and marry, and more are doing so, it creates a legal state-by-state mess more convoluted than a Wall Street Derivative, and you know how great those have been to the market.

Oh, and can we talk about divorce?

Marital Equality on the Federal level is the Man/Woman/One or the Other-date.