What influences the order of names in a couple?

[Kobi]

This came up on another thread and made me curious. We refer to our community as Butch-Femme not Femme-Butch. We usually refer to Medusa and Jack, not usually Jack and Medusa.

But lets broaden it. Why is it:

~Lucy and Ricky but Fred and Ethel?
~Meg Ryan and Dennis Quaid but Tom Cruise and <insert woman here>?
~ Howard and Marion Cunningham but Joanie and Chachi?
~Ross and Rachel but Monica and Chandler?
~Richard Burton and Liz Taylor but Liz Taylor and Eddie Fisher?

So, if you are thinking harder, think about couples in YOUR life, your family, your own relationship......is there a customary order for the names or are they interchangeable? Either way, what do you think influenced a particular order or an interchangeable order?

Is there a sexism component? A real or preceived power component? Does one just sound better/flow better than the other? Age? Tradition? Something else?

Inquiring minds are curious.