I always make sure in my more recent roleplays to specify sex rather than gender. While I'm pretty liberal minded when it comes to LGBT stuff, for the purposes of roleplay and quick visual identification, male or female is pretty much necessary because telling me something like androgynous or one of the 50ish other Tumblr-made genders won't really help out in that regard, especially depending on setting.
As so far as roles either sex play in a roleplay, honestly depends on the setting. As others have mentioned, historical real world settings are probably going to be majority male if it's combat oriented, and that goes in for some fantasy or otherwise fictional settings, as well... although that depends entirely on what setting we're talking about. Let's say you're doing a roleplay based around let's say Wonder Woman and the Amazons. Pretty big No Boys Allowed club there, or you're playing a high fantasy setting based around an elven culture that is more or less matriarchal or whatever.
But exempting all of that and say you're playing in a more or less egalitarian setting where males and females are on equal footing, I honestly go either way. I get an idea for a character in my mind, and I decide pretty early on if they're going to be male or female based more on feel rather than ability; the character could be either sex and still work.
I've found that so far, my female characters tended to have a harder upbringing or history than most of my male characters and are largely shaped by those events and how they overcame them. Two of my most played characters are female, one who is a ruthless, untrusting and determined warrior grew up in a broken home that didn't love her and she found a surrogate father figure in a mentor figure that taught her a lot of the skills she'd need to survive out in the world to find her brother, the other an idealistic young woman with brilliant mechanical skills who joined the military, saw combat and became traumatized from the experience and witnessing war crimes, and couldn't cope with life back home so she became a mercenary to try and figure her life out in a life that's somewhat familiar. While both have the whole broken family thing going on, that's pretty much par the course for most of my characters (male or female) and I am genuinely trying to write more characters with perfectly happy and mundane upbringings because the broken home thing is par the course for most roleplayers.
Here's a drinking game for you; go through any RP's character tab and take a drink for each one that features dead or abusive parents and surrogate parental figures and a drink of water for every normal childhood. I'll see you in the morgue.
For everyone who says they don't feel comfortable writing a character of the opposite sex because they don't feel comfortable with it, I highly encourage you to give it a try sometime. I put it off for so long because I honestly felt the same way, that I'd fuck it up or create a character that was more of a stereotype than a real person. I took the chance and ended up with one of my most rounded, fleshed out and believable characters that I love enough to have had art commissioned of her. Thing is, writing for guys and girls is pretty much the same process, just with minor differences here and there that make sense when you come across them.
Quoting George RR Martin in an interview,
'Yes, you're right I've never been an eight year old girl,' he says, 'but I've also never been an exiled princess, or a dwarf or bastard. What I have been is human. I just write human characters.'
โSome women hate the female characters,' he says. 'But importantly they hate them as people, because of things that they've done, not because the character is underdeveloped... Male or female, I believe in painting in shades of grey, all of the characters should be flawed; they should all have good and bad, because that's what I see. Yes, itโs fantasy, but the characters still need to be real.โ
It's honestly a pretty handy metric to follow. If you can write a real feeling male character, you're able to write a real feeling female character. Just treat them like they're a real person with independent hopes and dreams and what not and the rest will fall into place. Just don't emphasize the fact they're a female; they just are. Hopefully your mother doesn't go around affirming to everyone that she is in fact a woman doing woman things.
This is part of the reason I am not a fan of when roleplays put a sexuality section in the character sheet... why should it matter? Sexuality is just a part of who a person is, and that's something that should come up naturally in a roleplay through context, not hanging a sign around your neck saying "I prefer these kinds of genitals". I mean, unless your roleplay is sexual or romantic oriented to begin with, it shouldn't be a thing that's front and center. I'm all for romance in roleplays, but I rather see it unfold naturally between characters rather than being an advertised feature.
Likewise, it's the same thing for your character's sex. Last thing you probably want to do is emphasize ultra-feminine characteristics to try and convince the reader you are in fact playing a real woman who enjoys putting on lots of make up and is super proud of her breasts and cooking skills while fighting in high heels and so on so forth. It's not to say that kind of character can't exist or even be played well, but either the author is deliberately making a sexualized character who fully embraces stereotypical feminine traits or they're an alien who never had a mother, sister, or female human friend to relate to.
Honestly, if you like wearing sneakers, worn out jeans, eating pizza and playing video games and having fantasies of murdering boatloads of goblins with a claymore, chances are there's quite a few women out there who think the exact same thing. If you're worried about being offensive with playing a female character and coming across a situation where she'd probably like to put on make up, dress up nice, and head out... that isn't offensive or stereotypical. It's a part of who the character is. Likewise, maybe your character is really a nurturing sort with a soft spot for kids and small animals. Once again, it's a character trait that could go either way. If you want to play a badass warrior woman... well, what do badass warriors like and do? Chances are, it applies to either sex.
Point is, don't be afraid to try it. Obviously, the advice applies for women who are hesitant to play men, as well; just the specific stuff is interchangeable. Hell, the previously mentioned female character I had art commissioned of? One of her defining traits is she's an incredible mechanic who feels more at home in the guts of some machine covered in oil and grease than talking about overly personal stuff with the people she lives with and she pretty much is a high functioning alcoholic who swears a lot, keeps her hair short but styled, and listens to alien metal music for the sole purpose of keeping people away from her work area. Doesn't mean she doesn't enjoy cleaning up, wearing nice dresses, and heading out on the town, just her job and most of her interests happen to align with skills and career choices that society sees as typically male dominated.
Anyways, sorry for the wall-o-text, had a lot of opinions on the matter.