And what with 'meaningful relationships with women' also having not been invented yet
I read somewhere that the reason yaoi originated in Japan is because the condition of women in that country was (and still is) so much worse than in Europe and America (even when women couldn't vote in America and Europe, they still had far more rights than women in Japan), that the one and only way to portray an actual romantic relationship was for it to be between men. A relationship between a man and a woman would be more "master/servant" than "romantic", and a relationship between two women would lack power since neither of the lovers could really do anything in their society.
I'm not sure if that theory is correct, but it makes sense to me.