Let's put it this way:
You've no problem playing a Dalek, a Lizardman, a Dinosaur or a mindset/alignment totally in opposition to you, but being a woman/man is a problem?
They're not THAT different. Basic rules being: (and yes, these are massively stereotyped)
Male: Lead with your shoulders, Express anger, Compete in most things physical, Focus entirely on one objective, Only relax when that main objective is achieved, fights should be quick and brutal.
Female: Lead with your hips, Express compassion, Compete in most things social, Spread your focus, only relax when the background objective is achieved, fights should be decisive and can be long-lived in coming.
Gay men tend to lead with hips, relax for background and long term fights. Lesbian women lead with shoulders, relax after main objective, short term fights.
Alterations from this depend on social upbringing/situations - but can go across the board in most Patriarchal societies. Matriarchal societies will alter both types towards their homosexual qualities.
Technology based societies will split genders apart. Spiritual based societies will bring them together.
As is normal, there will be differences - but if you start at the stereotype and determine which way they split from it, you'll have the beginning of a character. (You can throw the same mindset at races...An American will tend to over-emphasize British standoff-ishness, a Brit will over-emphasize American expression, but once you get the hang of it, they're reasonably easy to portray)
Just think on basics to start with. If your female is a gun bunny, she'll be unlikely to use a mini-gun (as without dermal plating/muscle-weave, it will strain her body), equally the guy is going to use the weapon with the biggest damage code and probably name the weapon.
If all else fails, just ask/watch a female roleplayer. Surprisingly, they find it as difficult to portray us as we find it to portray them, so you can give them pointers as well.
