I've most often heard it explained as predominately-female fandom taking the objects of fascination which aren't like them (and may fascinate them regardless of the writer's sexual preference precisely because they aren't like them) and working with them by turning them into something they do understand (which doesn't quite defeat the purpose, because then all the interesting stuff which genderswap makes possible can then happen). Alternately, I've heard it described as a vindictive attempt to inflict male characters with the troubles of womanhood, but that's often seen as misogynistic. (Although, right now, as I sit and think about it, I'm not sure how fair that is.)