What is with everyone not understanding "libertarianism"? There are several definitions of the word with some being completely compatible with "socialism" (which also has diverse definitions). As someone already pointed out, there exist libertarian socialists (a term with which I greatly identify). So, while the chick was confused about libertarianism and socialism as they are understood in the USA, she is not incorrect to say that there exists libertarian socialism.
The problem is that "American" libertarianism is distinct and implicates a free market ideology/economic system. That is likely the place where the OP gets in trouble. He likely sees eye to eye with atheists regarding social issues, but then has problems when it comes to economic systems. I guess I would like the OP to outline what it is about his "libertarian" views that upsets so many of his atheist/liberal friends.
I really do not think that the OP should have that much trouble finding someone however, and am truly mystified about it all. It seems to me that the only hangup involves different views on economic systems, but atheists tend to be some of the most accepting people so it just doesn't add up in my mind. Undoubtedly, the pool of atheists that don't want children is fairly low, but is certainly not THAT low, especially in a large metropolis like DC.