Computer avatars

Computer avatars are most commonly photographic, artistic, or 3D representations of an internet user or a gamer's alter ego. Avatars can, at the very least, enable the user to experience what it feels like to be a different gender or age. However, it is not uncommon to find users taking advantage of MMORPGs to become fictional or historic characters. [1]

Gender Swapping
It is not far-fetched idea that there happens to be a lot gender swapping taking place online, especially in chat rooms. The reasons can be many -

(1) The user can experience what it feels like to be the opposite sex.
(2) A man may be able to get more attention by adopting the avatar of an attractive female.
(3) To explore or experience intimacy and romance from the same sex, and explore feelings of homosexuality. [2]

Ideal identities
Many avatars, mostly in games, are used as a means to obtain ideal identities. Users can experience what it feels like to have the most ideal physical characteristics. It can be as simple as the user adopting an avatar that is more muscular than themselves, or choosing an entirely different skin tone. The ideal self may or may not resemble the user in real life. [3]

Concealing original identities
There may be numerous reasons to conceal identity online, privacy being a major concern. With that amount of spamming and phishing taking place online, internet users have had to resort to extreme measures to protect themselves.

Adopting different avatars has also made it easier to broadcast hate speech, sexism, and racism, while keeping the user's original identity concealed. Cases of prostitutes advertising on Craigslist made their way to the news. Law enforcement, to beat the assailants at their own game, has used the anonymity of the internet to their advantage. "To Catch a Predator", an investigation on MSNBC, worked with the vigilante group Perverted Justice to convict adults trying to lure minors into sexual situations. The staff at Perverted Justice posed as minors in chat rooms and were able to convince predators that their fake identity was indeed real. Thus, at the end of the day, the playing field was level. [4] [5]

Footnotes
[1] http://www.wisegeek.com/what-is-a-computer-avatar.htm
[2] http://www-usr.rider.edu/~suler/psycyber/genderswap.html
[3] http://www.gamecareerguide.com/features/795/reinforcing_the_wall_hegemonic_.php
[4] http://ict4peace.wordpress.com/2009/08/11/online-identity-real-or-fake-for-a-reason/
[5] http://arstechnica.com/web/news/2009/05/no-erotic-services-no-problem-for-prostitutes-on-craigslist.ars