In a gay marriage, do both partners take each others names?

Like say Sue Crane & Kim Smith got married? How do they determine who gets which name? Or do they take each other's names...Sue Crane-Smith & Kim Smith-Crane, or do they flip a coin to see whose name comes first, since families share the same last name? I figure if it's a fem & a butch, the fem would get the butch's last name, like Rosie O'Donnell & her partner, but what about 2 fems? Or 2 butches?