If there's anything immoral about playing a character of the opposite sex, it would only be in using it to mislead others that you actually are something you are not -- but that's a separate issue. If someone assumes the gender of your character is your real life gender, that does not make you a liar -- it merely reveals that person's taste for jumping to conclusions. It's fruitless to rearrange any aspect of your life according to the assumptions and expectations of random people all around you, so why should other people's assumptions be an issue in roleplaying? Besides, it should be obvious to anyone starting up the game for the first time that anyone can make any sort of character they like, and therefore anyone you meet in the game could possibly be either male or female. There's no way to know for sure, and there's no reason it should really matter, unless you want to start a real life romantic relationship with them -- and that's a wholly different can o' beans.
Now that I've said all that, let's look at one other big and legitimate reason that people rarely think of for roleplaying a character of the opposite sex: there's actually something to learn from it.
Men and women tend not to understand each other very well. Some may brag that they've got the opposite sex all figured out and boiled down to a few key attributes, but chances are they're way off. We go about our lives as one gender or the other, always interpreting the actions of the other sex through our own differing experiences. Normally, there's no way to understand the gender-specific experiences of the opposite sex except vicariously, by listening to someone tell you about them. But with roleplaying, a door is opened where you are able to put on a virtual costume and play the part of someone you could never believably portray on a physical stage.
Of course in ages past, men used to commonly play the roles of women in theater, since acting was viewed as a man's work. The first viewers of Romeo and Juliet probably managed to temporarily forget that those were actually two guys up there professing their love for each other. In WoW, however, nobody knows what gender you really are, and in roleplaying it doesn't matter anyway. You can tell or you can keep it secret -- either way, your character is a way of interacting with others spontaneously while thinking from an entirely different point of view from your own.
Don't misunderstand me here. Roleplaying a character of the opposite sex is certainly not going to teach you everything you need to know about them. But it can open a certain doorway to a few experiences you might not otherwise have ever in your life, and provide some small but important insights for people who are interested in learning.
But in order for this to work, there are several important principles to follow. First, it's important to have a certain amount of distance between your character and yourself, as well as a certain amount of intimacy. He or she is someone you can relate to, care about, and enjoy portraying -- but whoever they are, they're not you. Your character is an extension of your inner self perhaps, but it's important to give them the freedom to think and act in ways that you wouldn't do in your own life. You already do this to a certain extent when you roleplay any sort of fantasy character -- roleplaying the opposite sex with any sincerity just brings it to a very subtle level, where you might rethink some of the things about them that you had taken for granted.
Second, it's important to put yourself in your character's shoes and constantly adjust your character's actions and ideas based on the experiences he or she is having. So, for example, suppose you are a man playing a female character, and someone uses the "/whistle" emote at you. You could just ignore this if you want to, of course, or else you can think about your character and how she would respond. Is this "unwanted attention" that would bother her? Does she view his whistling as a threatening desire for her, an advance from a man she has no interest in? Or does she view it as a statement of appreciation for her beauty, a gift with no strings attached? Perhaps she feels something else entirely? There's no one way to respond to such a situation, and a lack of familiarity with it may leave you feeling confused as to what your character would actually do. As your character experiences things, in a sense, so are you -- and as these experiences grow, your responses to them can get more and more sincere and natural. You won't ever get to the depth of understanding a woman has about womanly things, but you might get a better sense for them using WoW as a tool to put yourself in their place sometimes.
For women playing men, the situation is a bit different. You might find yourself accepted by other males as a natural member of their group rather than having to get past the perception that you are somehow other than they are. Chances are that no one will ask you "are you a guy in real life?" so no matter how you react to situations, people will assume you're a man unless you tell them otherwise. You could take advantage of this to shock their expectations by being every bit as feminine and girly as you are in real life (or even more so!) and see how they respond -- or else you could think about the things they're saying to you, what they mean to someone on the inside, and find yourself seeing a lot more about the ways men relate to each other. I know for my part, I would love to know a woman who roleplays a male character and actually desires to understand men better this way, rather than just assuming men are all really simple and there's nothing more to understand. I imagine this woman could have a lot of insights about the way men think, and it would be neat to see how she thought of men differently after her experiences.
Roleplaying the opposite sex can be tricky sometimes too, though -- there's no doubt about that. How do you approach roleplaying a romantic storyline? When is it important to reveal your real gender, and when is it best to just let it be a mystery? There are so many intricate details and approaches to these questions -- as well as boundaries that ought not to be crossed -- that it's really too much to cover in a single article. For now, just consider the possibility that maybe there are some insights and practical uses for this expression of the art of WoW.
Have you ever learned something from sincerely trying to roleplay a character of the opposite sex? Do you shy away from it for some reason (other than "that's just gross")? What are the major principles you think one should keep in mind?