I had one male player who preferred to play females, except they always came off as men in woman suits -- or near-feral she-wolverines who had sex at really inappropriate times and never wore shoes.
Had issues, that guy.
I think that guy got reincarnated into my gaming group somehow. Always plays female characters... and plays 'em poorly. As has been stated before - more charactatures than real people. He's getting better.. but it'll take some time.
Me, personally I tend to stick to male PCs just for convienance sake for the same reasons Darth Shoju mentioned - if I'm playing a female character I have to take a moment to think "Am I doing this right?" because the thought process isn't one I'm used to. If I don't take that little bit of time to reflect on how the character should act/react it just usually ends up being androgenous. As for characters themselves... lets see here..
-Have played female characters, successfully I'd like to think, in that they wern't fakey, flakey, or whoreish.
-Have played homosexual male characters, with the easiest note for that being to take a page from Bob the Angry Flower; the Homosexual Robot Cop strip: "But isn't he gay? Yeah, but my whole life doesn't revolve around me being gay, you know." Don't fonce about and be so camp that you'd make Big Gay Al blush, and gay male characters are easy to pull off.
-Have played a straight male character, but one who was a crossdresser. A fighter/bard who was raised as a female by his family due to the local corrupt lord bad-guy's edict that all first born male children had to be killed at birth (he was trying to dodge a curse placed on him). The character was a demure little thing, and for 95% of the campaign the other PCs didn't know that she was actually a he.
-Have played a lesbian amazon, nearly identical to the type Kahuna Burger mentioned, except that mine had the left breast bound down flat to the chest all the time rather than burnt off. Her outlook was basically that men are all well and good, but you didn't couple with one out of love - you did so to have children, which after they were born were his responsability. Other women were your comrades-in-arms, and occasionally your lover. Seemed only natural, you fight alongside this person day in day out, that you two may develop an attraction to one another. Besides, with same-sex relations amongst your warriors you don't run the risk of an accidental pregnancy taking one of the soldiers out of commission for months and months.
Heh, I like the girdle of fem/masc thing that Med Stud mentioned. May have to give that a shot. Would be amusing ^_^