Another thing you can be is a different gender than you are in real life. Some do this to experiment, some do this because they are roleplaying a character of a different gender.
It doesn't really matter why you choose to be something else - the point is to have fun and experience here in Second Life.
There seem to be lots of people who don't really "get" that, however, and show prejudice if someone isn't exactly what they are in real life. They also seem to think it's their "job" to "expose" someone if they aren't exactly like they are in real life.
Face it, people. Anyone can be Anything here in Second Life, and it's not YOUR job to "make sure everyone knows" who they are in real life. That's both a violation of privacy and just downright rude. Have some respect!
If the person is playing a male character, address them as male; if they are a female character, address them as female. If they are something else entirely, ask them how they would like to be addressed. Even if you know the person's RL gender, have some respect and address them as they present themselves - not based on what you know.
Even here on Livejournal some folks present themselves as other than their RL gender. There is nothing wrong with that; address them as they present themselves.
What prompted this was an exchange with someone who is now banned from my journal. They say in their own journal profile that the journal is for someone they play on Second Life, they prefer to be addressed as that person, and anything real-life is irrelevant - and yet, they turned around and addressed another SLer by real-life gender, even though they are presenting themselves as a character of the opposite sex.
That, my friends, is rude, and approaches hypocrisy.
Have the respect to address someone as they present themselves - especially if you expect others to afford you that courtesy.