I've long been a defender of [blank]-only spaces in general. Most of the safe spaces that are created for any reason other than "to make a point" accomplish some pretty great things. I don't even agree with the argument that their creation should be limited to minority-only or oppressed-only groups, as long as there's a thoughtful reason behind why the space was started. For instance, I would support a forum provided for white individuals to talk about issues of race. There are certain things that can be said and done in such a group that can't happen in a mixed group (or worse, in a group that ends up with whites greatly outnumbering people of color). I wouldn't of course support a whites-only country club or somesuch, for obvious reasons.
I also don't think that the organizers of said safe space should have hard-and-fast rules regarding who gets to define as eligible, or should personally determine who is [whatever]-enough to qualify. A friend, who is biracial, once told me about being in a people-of-color meeting in college, seeing a white-appearing woman come into the back of the room to use some journals, and seeing the group leader tell her "sorry, this is people-of-color-only space right now." I don't think that sort of thing is appropriate, because it isn't the place for the leader to assume that the woman couldn't be biracial, or white Hispanic, or have some other entirely respectful reason for being there. I could understand if the woman had come in at the beginning and maybe looked confused about which meeting it was, that someone in the group might approach her apologetically, explaining that they didn't want to assume or to define her, but wanted to make sure she was where she intended to be. After that, it isn't up to anyone else to decide that she isn't "of color enough" to be there (Michigan Formerly Transphobic Womyn's Music Festival, anyone?). Unless, of course, she's coming in and explicitly stating that she's white, doesn't identify as being a person of color in any way, but still feels she should be included. Which brings us to my main reason for defending safe spaces:
Someone who's never drunk alcohol and never thought about doing so wouldn't be kicked out of an alcoholics-only AA meeting, but most of us would wonder why exactly they were there and/or implying that they define as an alcoholic. If they're sitting there stating that they never drink alcohol but just want to make a point that AA is discriminatory, others are surely going to feel that they don't have much of a place in an alcoholics-helping-other-alcoholics meeting, and might feel offended and violated. Not because non-alcoholics are bad or unsafe people, but because you gotta question why someone would want to come to an alcoholics-only meeting and state that they don't drink and don't intend to. If the person is just interested in helping alcoholics, they can become a counselor or volunteer at a detox. If for whatever reason they wanted to just sit quietly and listen and be supportive and state that they're an alcholic but not share anything more than that, no one's going to mind. But if they insist on stating that they don't drink, they're really kind of violating the space that alcoholics have created for themselves.
Same with someone who goes to a queers-helping-queers program. If they want to explore the queer community and are comfortable stating that they identify as queer but not sharing much else, that's fine. They won't even be asked why specifically they're queer -- just being there and being respectful is all that's expected. If they feel that they're queer because they sometimes have thoughts of feeling not-manly-enough or whatever, no one's going to tell them they aren't "queer enough." But if they go into a queers-helping-queers program and tell everyone that "hey, I'm not queer, and I want to make sure everyone knows this, but I want to be part of your queers-helping-queers program anyway," you gotta wonder why. If they just want to help queers, they can become a counselor specializing in working with queer people or they can volunteer for a civil rights group or GLSEN or something.
(Please note that this is not in any way to imply that being queer is analogous to a disorder -- just an attempt to explain why many creators of safe space view it as creepy to violate it in certain ways.)