Joseph, from Engage: Conversations in Philosophy, has a great post up called Can men be feminists? Joseph discusses a colleague of his, Dr. Lani Roberts, who is of the opinion that a man is/can be considered feminist "if he's broken the male bond in defense of women."
On the surface, I can't really disagree too much with that criteria. After all, if I, as a man, never do anything to risk my position of comfort- if I never challange patriarchical standards and norms or point out sexism when I see it... what good am I doing? If I'm only willing to discuss feminism with other feminists, my contribution would be pretty minimal.
Even this, though, is going to run into a wall eventually. Ignoring the question of how often someone has to break that bond, there's the problem of identity. Identity is complex, and the problem is figuring out where internal and external identity match up, when you're talking about identifying with a socio-political movement like feminism. If I'm a firm believer in the tenets of feminism, but don't do anything, my internal identity may conflict with people's external perception of my identity. I might believe myself to be a feminist, and call myself one, but other people look at me and think "But he doesn't seem to hold any feminist beliefs. What has he ever done to support the movement?"
I think that this difference also illustrates what I think is a more controversial issue. The question over whether men can be feminists doesn't really seem that controversial. There's some debate about it, but, from what I've seen, most people agree that men can (even if some think it's rare) understand and work towards feminist goals. In other words, the debate isn't generally about whether men can be feminists- it's whether men can be called- or call ourselves- feminists.
That's a little more difficult to discuss.
Luckily, lot has been written about the naming of men within the movement. Chris Clarke, of Creek Running North, has a really compelling take on the subject called: Why I am not a feminist. It's a tremendously well written piece (so, you know... go read it). Clarke considers himself a sympathizer, or, at his best, an ally. His argument against considering himself a feminist is that he is "a member of the class against which feminism is aimed." He compares calling himself a feminist to a white activist calling hirself Chicana as a result of showing political support.
The comments in that post are really interesting too, and there are a number of counterpoints to be made to Clarke's post. In particular, I was pleased to see that someone pointed out what I was thinking- there's a distinct difference between claiming membership to a group based on socio-political activism, and claiming membership to a group based on something like ethnicity or racial identity. Claiming to be an abolitionist or a feminist is very different from claiming to be a person of color or a woman.
There are more great thoughts on Clarke's post from Piny and Jill over at feministe, in Self-Appointed, and from Ampersand over at Alas! A Blog in the post Should men be called feminists?
Ampersand mentions what I think is a pretty important point, and one that I think most of us get (and which is probably the reason why this is largely a non-issue, most of the time): there's nothing to be gained by trying to force people to call us feminists. There are some groups that are fully supportive of men who call themselves feminists, but there are also groups that are troubled by it.
I consider myself a feminist, and if anyone asks me, I have no problem taking the title. That I consider myself a feminist doesn't mean I can force my way into women's spaces, though. There are some places that I'm just not welcome, and that's okay. I absolutely understand the desire and need for women to have spaces where men- even men who are allies- aren't necessarily welcome. I also understand that there are women who will be uncomfortable with the idea of men calling themselves or being called feminists, for the very reasons that Clarke and others point out. And that's okay too. Part of being a feminist or feminist ally is understanding why that might be so, and understanding that the label doesn't really matter.
It goes all the way back to the original qualification, in a way. Feminism isn't a tag you put on a shirt- it's something you do, and I think that most of the men who are working to "get it" or who are willing to wear the label of feminist in public and outside of feminist circles probably don't really care about the label. My guess is that the sort of guy who's willing to tell his drinking buddies "Hey, that was sexists" or is willing to out himself as a feminist to them isn't going to be upset if he's interacting with feminists who would rather he call himself an ally.
From a personal perspective: I tend to think that my calling myself a feminist and being open about it has value. I think that being open and proud of my association with feminism does several things: First, as others have noted, it's a way of grouding discussions of sexism with other people. It makes it clear that I'm not just arguing for the sake of argument or playing devil's advocate- I'm saying things that have meaning to me and that I really believe. Second, I think that it helps fight the negative stereotypes that are pushed about feminism. Feminism takes a beating in the popular media as a movement that "hates men". At my last job, a woman heard that I consider myself a feminist, and actually confronted me on those grounds "But, you're a guy, how can you hate men?"
And, at the risk of treading into dangerous territory, I think that feminism, as a movement, won't succeed until at least some men get on board (this is in no way an argument that feminism should be friendlier to men, or that men should be given passes, etc). I don't think that it's men's involvement that will make it succeed, but, given that men and men's complicity in (and benefiting from) patriarchical systems tend to be the things creating the problems that feminists are trying to fight, there's a point at which at least some men will have to get on board (or at least get out of the way?) in order to overthrow those systems.
To that end, I don't think it matters if men actually call themselves feminists- but having some men who openly support feminism and are willing to call themselves feminists, I think, helps. It's more important for men to be interested in being feminists than taking the name.