I think that the option should be available for every party in a marriage to choose which last name to take, whether that be that one party takes the last name of another (regardless of sex), everyone keeps hir own last name, or everyone changes to a completely different last name. This is a situation where I really wish that there wasn't a social protocol for it.
I think that in heterosexual marriage the wife taking the last name of the husband has its roots in sexism, as it's a leftover from when marriage was the passage of a woman as property from her father to her husband. Her changed surname reflected the change in ownership. In modern western society, I think it may be indicative of an expectation for a wife to subsume her identity into that of her partner, but I'd like to think that in more cases it's just wanting to share a last name and going with tradition. It's also significantly easier in most states for a woman to take her husband's last name than for a man to take his wife's last name, which I think is sexist.
I haven't been made aware of any trends regarding last names in same-sex marriages, but I'd be interested to know what most same sex married couples do. For every couple (or group, in a consensual group marriage situation) it should be a decision made together that everybody agrees on. Certainly nobody should be able to force hir spouse to take hir last name.
On the other hand, I'm deeply suspicious of marriage as an institution. If I ever decide to get married, our surnames will be something we'll discuss and come to a consensus on; everybody should be able to choose for hirself, regardless of sex.
If I bang my head against a brick wall five times and get five lumps, why am I surprised when I bang it a sixth time and get a sixth lump?
"Isn't it funny that the only time your race or gender is questioned is when you're not a white man?" - Wanda Sykes