Any remotely thoughtful, intelligent, observant person with decent general knowledge can disguise his or her gender online with relative ease and a low probability of being unmasked.
consumagenerica, above
The above quote is probably very true - in English.
But what about other languages? Many other languages are much stricter on differentiating between masculine and feminine in much more than just the pronouns. I suspect this would make it much trickier for someone to try this in, say, French ("mon" or "ma" for "my", depending on gender), and even harder in say Hebrew where the word for "I" is always "Ani" but the verb following it changes depending on whether you're a guy or a girl.
After 15 or more years of speaking a language, how easy is it to temporarily flip into the other gender, without making a slip that would give yourself away?
Something to think about!
A few people have pointed out my mistake in French - "ma" / "mon" depends on what you're talking about, not your gender. But my Hebrew is better than my French and that one's definitely right!