Traditionally it's quite normal for an author to speak for characters of a different gender to his or her own. Gender is usually inferred right there on the cover by the name of the author and then confirmed on the back with an author photo. Once the story is started it's quite alright for an author to slip on the persona of male or female and we are all quite happy here with our coffee, because we know we're being told a story. This has not applied equally over all genres however. Romance, deemed to be of interest only to women, often has male authors writing under female pseudonyms or gender non-specific names to leave the reader to make the assumption. The same has applied, in the past, to women writing in the Mystery genre, and I'm assuming it still could happen in the Western Genre.
It's quite different when an author leaves the safety of solitary authorship and joins the ranks of the collaborative writer, a group that has roots and relationship to role-playing games. Why it should have evolved to be so I'm not exactly sure because I often played a fiery sultry vixen half-elf enchantress who liked to wear skimpy clothes and could use a knife like a ninja assassin, but then a lot of guys got used to playing female back in the early days of D & D when nary a female dared show her face in that pimply testosterone heavy crowd (I see a relationship to Elizabethan theater here). Things are very different now, I know, but I'm talking over twenty years ago. Eventually women were allowed into gaming, I'm glad to say, and the last time I played was Vampire the Masquerade where half the players were women. The Vampire game involved a lot more acting and story telling, which I found even more fun than all that dice throwing and long character sheets and endless discussions of percentiles and weapon weight, but it seemed to involve less cross gender play. Perhaps because you were speaking the lines out loud and didn't want to appear foolish - but certainly my group tended to stick to their sex with the exception of the game master and their entire cast of thousands.
When a writer or role player comes online and logs into a site as his or her character there seems to be that same sense of gender association, but there begins a merging of the lines between fact and fiction. Without the visual cues all a person has to go are the written words. Thus when a person signs on to Pan Historia, for example, they are free to commit themselves to a fictional persona, of whatever gender, and then be accepted at 'face' (avatar, bio, etc) value. Meeting and greeting new people as they explore Pan we move people towards this association with character and discourage the standard sex and age questions that prevail in other forms of online social site.
The association with the character is so strong in the role play and related collaborative fiction world of Pan Historia that people often do not remember that the person playing that character and writing those stories is the author. The convention that the author can do as they please in writing for male or female breaks down and the character becomes the author. It doesn't seem to cross into other areas of the character like profession, for instance. If I'm writing a ninja mage (going back to my elf analogy here) no one assumes I'm really a ninja mage or elf. But if the first time they met me I was also presented as a female elf, the assumption would rest squarely that I was a woman in real life. I have to admit that my example breaks down a bit when I played a sadistic killer for the first time. I did have online friends decide I must be a sicko. I consider that flattery; I must have done a damn fine job of writing.
The interesting thing is that, quite obviously, lots of people are writing characters that are not their own gender online, or playing them and everyone knows that. The majority of World of Warcraft characters are not male, yet the majority of their players are. Some players are perfectly open in their forum and other out of character communications and some players choose to remain in character. People turn a blind eye until someone is 'outed'. The whole issue seems to have become less fractious with time as more people realize that it happens all the time online (and try it themselves), but there are still a surprising number of stereotypes and thus a lot of reasons for an individual to prefer not to 'out' themselves as one gender or another in their writing partnerships. We think it shouldn't matter - and it seems to me it really shouldn't - but gender identification is such a fundamental to the bone social programming that we seldom question our gut reactions.
For instance my female character that I had for many years as taken at face value as a strong woman while people assumed the writer was female, but later on people shifted their views and I found her far less successful to write with, including interactions with other collaborative writing partners. The example that comes to mind was her love affair with a male character where suddenly they were fighting and not having a passionate love affair once the writer realized I wasn't a proper woman. In effect his discomfort at writing romantic fiction with a male writer altered the way his character behaved to the point that the story changed radically. This was not an isolated incident.
One positive reason, though, that I can cite for total immersion identification with your characters of the opposite gender is what you learn about gender from it. It can only enhance your writing to really start to relate and understand characters of opposite sex. Of course when a male author sits down to write a novel with a female protagonist (or vice versa) they are doing exactly that: slipping into the skin of that character. It should be no different with any form of writing ultimately.
In online writing relationships I guess the best maxim is: do no harm but have fun and explore new things.