As promised, here are the responses (from the comments section, Facebook, and Twitter) I found most insightful and amusing. Let me know if I missed any good ones!
Culturally, a man could go off to hunt for food or fight a war or chase a Golden Fleece, and the woman was at home with the kids. If he got eaten or captured or ended up hanging out with Sirens, the woman had to figure a way to raise the kids. On the flip side, if a woman died in childbirth or from consumption, the man remarried (often a younger woman) or hired someone.
In short, while a man's ability to succeed in the provider role may or may not have been tied to his ability to share with others and rely on a larger community - women have been well aware that a lot of the forces controlling their lives are out of their hands, and are more likely to seek communal security.
Another factor - men functionally have unlimited bullets to shoot to expand their presence in the gene pool. Women's ability to maintain their lineage depends more on successfully rearing a smaller number of childern than in maximizing the number of children they bear (which can have a negative effect on survivorship and quality of life and opportunity). I think it's easier for some male minds to embrace the concept "the weak just shouldn't survive" than for women to. Hell - I'm 53, and if our two children were killed in some accident I could still start a new family. That option isn't available to my wife.
The correct answer here is that women don't reject libertarianism, but the amorphous bag of self-contradictory ideology, religious dogma and incoherent chauvinism which a hard core of recidivist conservatives are trying to pass off as libertarianism.
As a female ex-Libertarian, I think there are other reasons.
An important one is that a necessary part of a Libertarian utopia, one that my 20-some self wasn't thinking about yet, is that all indigent care is done in a familial setting. Unless one is wealthy enough to contract it out. We hear it now as Tea Partiers proudly announce that they take care of their own.
When I progressed in life enough to do some observation and think about these things, I could see that:
a) Guess who does all that care in a famillial setting. Yep.
b) Few are wealthy enough to contract it out, and I had already started to notice that who becomes wealthy, is not all that often who is hardworking/intelligent/creative.
Another important reason is that we ladies have, if we have enough decades of memory to be of age of any influence, are quite aware of what an unfettered private business would do regarding opportunity, and that was none for us, except fetching coffee. Or teacher/nurse. Or identifying ahead of time and getting hitched to that brilliant captain of industry. It took growing cultural awareness and some governmental interference, to make it at all possible for me to become an engineer, and early on in the '70s and even '80s there were rocky parts on that road as it is. That latter aspect, for awhile, fed my self-important feelings of having been a trailblazer against the odds, but, being a trained observer too, the former aspect eroded my Libertarianism.
And other general observations, maybe women being more attuned to the social makes a difference too. It just doesn't fit with what real people do, in real life, no more than the Bolshevik 'Soviet man' is real. When little Suzy and little Johnny have competing lemonade stands, a lot happens other than competing with each other to make better cheaper lemonade. (Just for starters, but for the law, Suzy may take a contract out on Johnny, so they collude ...)
Women are less likely to be one-dimensional in the same way that men can be. Men are more prone to build systems and models in their heads that they mistake for reality. They can do this as adolescents because that period brings out their obsessive tendencies. Something similar happens in midlife when many men feel frustrated and are looking for answers. In both cases, they're often in their heads and not very grounded. Women are just less apt to be like that.
I'm reminded of a comment George Will (not necessarily a libertarian but you get the idea) made when Princess Diana died. In response to the overwhelming outpouring of emotions, George remarked on TV something like "I'm surprised that people are so emotional over this; they must feel something strongly." Not only disconnected from reality, but seeing the world through a broken model. It's like he was saying "my model of the world is broken, and I'm confused."
Or take Alan Greenspan's response to the 2007-08 financial crisis: "I made a mistake in presuming that the self-interest of organizations, specifically banks, is such that they were best capable of protecting shareholders and equity in the firms ... I discovered a flaw in the model that I perceived is the critical functioning structure that defines how the world works." Well, duh. This is a statement that could only be made by a person whose perceptions and experiences of life are very incomplete.
Men are just more likely to believe their own BS--to build castles in the air and think they're real. We can tend to be very overconfident and blind to their lack of awareness.
Brian from Facebook:
This is entirely conjecture, but I would think that women are most underrepresented in the crazy wing of the libertarian party - the wing that doesn't just want to severely limit government but instead turn America into a anarcho-capitalist utopia. There's a strand of that thought that is distinctly masculine, the strand which says that this would be a world where a person is completely individual and succeeds or fails only on their personal strength. Men are more likely to have unrealistic expectations that they would succeed, rather than fail, in this Hobbesian state. It's really the same reason more men than women enjoy action movies, because men watch Die Hard and think they could be Bruce Willis's character, while lots of women just think it's a silly fantasy.
Kate from Facebook:
Women generally are more vulnerable and recognize the need for social safety net programs provided by government that they or someone in their family have or may use at some point in the life.
Jeff from Facebook:
Motherhood. It is hard to be a "rugged individualist" when caring for babies who can't fend for themselves. Libertarianism is the philosophy of young men with nothing to lose.
And a few choice tweets: