See subject.
The question needs more clarity. For example, what is a “same-sex transgender couple”?
A couple where both had organs of one gender, and then had surgery for organs of the other gender, so that they are now both the same sort of transgender person?
Or a couple where both had organs of one gender, and then one of them had surgery for organs of the other gender, so that they started out as same-sex, but are now heterosexual?
Or a couple where they had organs of different genders, and then one of them had surgery for organs of the other gender, so that they started out as hetero, but are now same-sex?
There are probably other possibilities too, but I’m getting dizzy.
NPR had a bit on this just last night. The Census Bureau estimates around 300,000 couples. But that is on self reported data and may included a lot of people who aren’t actually legally married. And who knows how transgendered folks would respond to that question? Are you SS married based on your original or your chosen gender. And given that somewhere in neighborhood of 0.5% of the population is intersex in some fashion or another (many not even knowing it), how would they report?
Add on to this the fact that many of the states with legalized SSM have adopted a gender neutral marriage form. So we can’t go there for data. I don’t think we will ever have a real count in the US. All we can say is likely hundreds of thousands.