Here are some reasons that a straight, married feminist/womanist woman might have taken her husband's name:
1. Because she was not a womanist/feminist when she got married.
2. Because it was a huge point of contention with her in-laws, or maybe even her own parents, and she was picking her battles.
3. Because a name change makes it more difficult to be found by a violent ex, a stalker/rapist, or anyone else by whom a woman might not want to be found—and a marriage-related name change is easy and doesn't create a public court record.
4. Because she or her husband immigrated for the express purpose of their marriage, and proving that they are a "real" couple to a government still steeped in patriarchal traditions is made significantly easier if she takes his last name.
5. Because she works in a field or at an employer or in a location where not changing her name risks revealing an ideological leaning that could affect her career or target herself/her family for ostracization.
6. Because her maiden name was her father's name and keeping it did not feel like any more a rejection of the patriarchy than taking her husband's name did, and she liked her husband's name better.
7. Because her maiden name was her father's name, and she likes her husband a lot more than her father.
8. Because her family was abusive and her husband's family is wonderful to her, and she wants actively to become a part of it and feels taking their name is a symbol of that joyful joining.
9. Because she and her husband want the same last name, but the law makes it infinitely easier for her to change her name to his than for him to change his name to hers, or for both of them to choose a new name they share altogether.
10. Because despite knowing it comes from a weird, fucked-up patriarchal tradition, there's still some weird, fucked-up place inside her that likes the idea of taking her husband's name—and no feminist/womanist lives a life free of compliance, consciously or not, with weird, fucked-up patriarchal narratives and expectations. But unlike privately calling another woman a bitch or playing the role of Exceptional Feminist with a group of male coworkers or secretly doing all the housework in her own home, the name thing is there for everyone to see and question, every day of her life.
This is hardly a definitive list. Not everyone who reads this selection will consider each (or any) item a legitimate reason for a woman to opt to take her husband's name. Still, few of us would feel inclined to directly tell a womanist/feminist woman who's survived and escaped a profoundly abusive family of origin and found a wonderful partner whose family she adores, and who adore her right back, that her desire to take their name is a betrayal of The Sisterhood.
Few of us would directly tell a rape survivor, whose attacker the justice (ha) system declined to prosecute thus allowing him to continue to stalk and harass her, that she's a traitor to feminist kind if she opts for a quick and quiet name change upon getting married.
Few of us would directly encourage a woman whose immigration status (or whose husband's immigration status) could be imperiled or delayed or made any more difficult than an already-labyrinthine process to prioritize her name over her entire future.
Yet that is most assuredly what we're doing every time we publicly castigate or question women who have taken their husbands' last names—because there are reasons, not always evident and none of our fucking business, for that choice which can and sometimes do trump political statements on a personal, individual level.
This is not to argue that taking one's husband's name is inherently a feminist choice (although I'm not sure it's inherently not a feminist choice, either, depending on the circumstances). It is merely to say that we cannot (and should not) axiomatically assume anything about a woman who has taken her partner's name, rendering this yet another subject on which the casual passing of judgment is a pernicious affair indeed.
Quite evidently, we each have a responsibility to think critically about our individual decisions, and not pretend they happen in a void even when we make choices for no one's pleasure or security but our own. just because one is doing something for herself doesn't magically turn it into a choice without cultural implications.
But it's eminently possible to critique the culture in which individual choices are made, and the cultural narratives that may affect our decision-making processes, without condemning those individual choices. Or the womanists/feminists making them.
Not every feminist/womanist will make the same choice, nor should they be thus obliged in order to prove feminism's value. Feminism has sufficiently demonstrated its own worth by providing that spectrum of choice in the first place.
And even though not every one of those conceivable choices is implicitly feminist, having a choice is evidence of feminism's reach.