The main problem for a lot of men is that their main relationships with women are sexual.
This means that at least an unconscious level the female characters they write end up sexualized - hence why women characters often stop to think about their breasts, or wonder how sexy they might look.
And if a woman character ever needs something to have struggled against to define them, then it's almost always sexual violence they've had to endure and survive (and even continue to do so). Almost never for male characters - because the writer doesn't define them in sexual terms.
Hence why I recommend swapping out character genders - because once you start doing this, it's easier to recognize and challenge such assumptions and change them. And once you do free characters of such restraints, it's easier to develop them better as individuals - as people.
Men just don't realize how much they are defined by their own gender, because the society we live in treats it as a normative experience, and rarely challenges it.
You know, for a bit, I thought we were just going to disagree on this issue, but this last post got me thinking.
There is a big problem with oversexualization of women in nearly all forms of fiction, SFF not excepted. And, unfortunately, I think you are right about the cause. Many men simply do not cultivate platonic friendships with women, and this leads to a massive lack of understanding about women in general, as well as a significantly misinformed perspective of female characters in fiction. So, from the context of understanding if a female character is sexualized, then yes, swapping genders could be a good exercise. Better, though, may be to make some platonic female friends, but in lieu of that...
That said, I don't think I could ever get behind the idea of there being no difference in any sense between male and female characters, so that a gender swap would be useful for other sorts of checks. Then again, I also don't see male as a default gender, so perhaps I look at the world differently than most. But, living in a house of women and comparing it to the house of men I grew up in, there are, decidedly, some differences. For example, I have never, once, considered my sleeping attire based on their aesthetic value (it is strictly a combination of comfort and temperature), but my nearly 6 year old will not wear pajamas which aren't pretty, or at least cute. No. Matter. What. I am convinced that she would maintain this insistence under the threat of death. But, what many male writers don't seem to understand is that "pretty" and "sexy" are two radically different descriptors, often with different audiences and objectives in mind. Obviously, my daughter couldn't care less about the latter, but the former? Life and death.
When it comes to fiction, we shouldn't shy away from the legitimate differences, even though we absolutely should shy away from the oversexualization that often occurs (unless that is what we are exploring, of course!). The goal, ultimately, should be realistic, believable, relatable characters who are just as complex as the species which inspired them. It seems to me that this requires non-accidental character traits, including gender and/or sex. I could be wrong, and I welcome the correction if I am. But, I like characters that remind me of real people... neither androgynous or sexualized.
On that note I wonder if you could train an AI (using language analysis) to differentiate between fiction written by men and fiction written by women - I wonder how much structural and tonal differences can be attributed to gender rather than specific writer.