|Via Flickr (Elephant Gun Studios)|
First thing first: people are people. It doesn't matter their gender identity. People are people first. Everything else is window dressing that's draped on us (and occasionally pinned or stapled on us) by society; we may embrace said dressing or not.
The trick to creating believable opposite gender characters is to....well, create believable characters. If you're trying to create a character based on flat stereotypes (the "typical man" or "typical woman"), they're going to fail.
How about an exercise?
Character 1 walks into their apartment and stops outside their mostly closed bedroom door, peering through the gap. Their partner is in bed with Character 1's coworker. Bile floods the back of their throat and they take a quick step back, then another, all they way out of the house. They get in their car and drive off.
Character 2 walks into their apartment and stops outside their mostly closed bedroom door, peering through the gap. Their partner is in bed with Character 2's coworker. Character 2, as if on auto-pilot, turns and goes swiftly and silently to the kitchen. They pick up a knife. Just as silently they return to the bedroom and shove the door open so it hits the wall with a slam, making the couple in the bed jump and scream. "What the fuck do you think you're doing?" they say, entering the room, knife held at their side but clearly visible.
Character 3 walks into their apartment and stops outside their mostly closed bedroom door, peering through the gap. Their partner is in bed with Character 3's coworker. Character 3's face heats, their head fills with static, time seems to slow. They pulls the cell phone from their jacket pocket and start recording, making sure to get the faces of their partner and the other person on camera. They set the video to upload to various social media sites. Then Character 3 pushes open the door, shouts "Say Cheese!" and snaps a still photo. This will make a great addition on the company's Announcement Board.
Which of these characters are men? Which are women? They could be either or neither. They're individuals, with distinct reactions to a situation that any person could share.
Instead of worrying about making a character who's a man, or a character who's a woman, stick them in a story, wrap a plot around them, and ask the truly pertinent questions. What are they working for? What haunts them? What do they want? Who would they die to protect?
When you do that, you'll get a character worth caring about. And any trappings (or subversions) of sex or gender can be added later, if or as they're pertinent, to make the character more real.
I say that last part, because we live in a society built on the idea that men do this and women do that; a society that doesn't often recognize options beyond the binary. So writing characters completely emancipated from the social baggage isn't entirely realistic. (Unless, of course, you're writing in a different kind of universe, in which case, as we all know: rules are meant to be broken.)
Bottom line: be wary of stepping into cliches or stereotypes.
As Mette Harrison writes:
Women, despite the sense of awe and fear that some beginning male writers seem to view us with, are actually a lot more like men than you think. As a culture, I think we have codified certain gender stereotypes to a point that is ridiculous and actually harmful to men and women.
Men are not all unable to listen, unable to ask directions, good drivers, bad at cooking, always thinking about sex, clueless about fashion, and unable to engage in deep emotional conversations.
Women, by the same token, are not all obsessed with their hair and makeup, worried about how many calories they are eating, thinking about how their butts look in this pair of jeans, helpless when it comes to math, illogical, and only interested in romantic comedies as movies. Men, and not only gay men, share some of these characteristics. This is perfectly normal and healthy. The characters you write, whether male or female, should never be examples of only-supposedly female characteristics or only supposedly male characteristics.