A Man. A Woman. Just Friends? - NYTimes.com: There’s a history here, and it’s a surprisingly political one. Friendship between the sexes was more or less unknown in traditional society. Men and women occupied different spheres, and women were regarded as inferior in any case. A few epistolary friendships between monastics, a few relationships in literary and court circles, but beyond that, cross-sex friendship was as unthinkable in Western society as it still is in many cultures.
Then came feminism — specifically, Mary Wollstonecraft, the mother of feminism, in the late 18th century. Wollstonecraft was actually wary of platonic relationships, which could lead too easily, she thought, to mischief. (She had a child out of wedlock herself.) But she did believe that friendship, “the most sublime of all affections,” should be the mainspring of marriage.