What @punkarsenic and @LordVincent said is true. I think the gender people perceive you as has everything in the world to do with posture (and a little to do with voice).
I am a woman, only 5 feet tall and very small-boned, but I had to play a man in a college play I was recently in. I was surprised to find out afterward that a lot of audience members had thought I was a man until I spoke to them after the show. I was extremely pleased, but still surprised.
What they didn't know was how hard I had worked to pull off the male persona onstage. For months while I rehearsed for the role, I practiced deepening my voice incrementally, and avoided sing-songy inflections (keeping my voice slightly more monotone). I also practiced "masculine" postures and mannerisms. For example, walking with my legs wider apart, my head held higher, and trying not to sway very much. My hands in my pockets sometimes, or just keeping them loose at my sides instead of keeping them stretched or tightened like I normally do. And when I sat, I leaned back in my chair and spread my legs slightly. If I crossed my legs, I put my ankle on my knee and leaned back with my arm on the chair behind me.
There was some more stuff I practiced, as well as wearing a back-brace, chest-binder, and a suit to make myself look more masculine. Lol, never doubt the power of makeup either. I thickened my eyebrows and contoured my face to make it look like a man's. Never thought it would have worked with my baby face, but it did, haha.
Anyway, a lot of this can be applied to being read as a woman too. The way you walk, sit, talk, move your hands, hold objects, move your head, everything. It can all change the way people perceive you. I hope some of this advice can help you.