Is writing across gender essentially harder than other examples of the writer's need to make an empathetic projection into someone not him/herself? I'm not sure. Possibly it is. But that projection is so important to what reading and writing novels is about that I'd hate the thought of problematizing it too much in the case of gender.
Atonement pops into my mind as an example of a very successful piece of cross-gender writing, and I'm sure there are lots of others. An example of it that I hated was Miss Smilla's Feeling for Snow, which I think at one point has the female character loooking in the mirror and thinking how goddamned sexy she is, in a way that just felt like the male writer allowing his perception of her to be presented as her own self-perception.
Actually, perhaps that's what does make men-writing-women especially hard: our society institutionalises men's projection onto women of men's own desires. To take the most horrible examples, when men see a woman as attractive and call her a whore, or when men feel frustrated by a woman and call her a cockteaser, they are misperceiving something relational (their reaction to her) as something intrinsic in her; they don't see her they see themself in her.
That is a reduction of women to (a small subset of) their relational properties rather than something 'in and for itself', and so the creation of a fictional woman in a male novelist's imagination is more likely to fail to generate something plausibly autonomous?
I still want male novelists to attempt it though.