I see this question getting asked a lot on photography forums, with new laws coming in to legalise gay marriage, wedding photographers are suddenly thrown into a panic as they struggle to fit all their planned poses to people. "Who is the dominant one? Who is the guy and who is the girl?" They cry.
The correct answer is:
Photograph same sex couples in the same way as heterosexual couples- let them pose together as they would naturally interact. If you ask two people to hug, you will often find they like to come together in their own way that they already know very well, it's almost automatic, then you can adjust the pose from there so it works for the shot. If you allow couples, both gay and straight, to interact in a way they are used to they will be way more comfortable and relaxed and thus easier to photograph.
Stop assuming there must always be a butch or effeminate person in a couple.
Stop assuming they can't be equal.
That applies to both same and different sex couples.