Carlos may be a man, but he's a troll who finds it funny to pretend he's a girl and then drop the Bridget when Bob has something invested in him actually being Alice.
Carlos is an epic scale Munchkin, and he has discovered a powerful Game Breaker dynamic that allows him to PWN n00bs with extreme ease. However, it only works if playing a particular female-only class.
"If I'm going to be staring at someone's butt all day, it's going to be a girl's."
Carlos is a cop pretending to be a prepubscent girl as part of a sting operation.
You can be whoever you want to be online, and Carlos wants to pretend to be a woman, dammit.
Alice meets Bob on-line, who turns out to be her (female) best friend Danielle in real life.note The opposite of a G.I.R.L. - Girl in Real Life
Alternatively: Alice pretends to be a guy online, which is kind of awkward for Bob when they try to hang out off the web.
Or, Carlos pretends to be a woman in real life, but can act as his real male self online.
When they finally meet Bob is relieved to see that Alice is a girl in real life.
Alternatively: When Bob goes to meet Alice, he sees someone in a suit standing where she said she'd be, and expects that it'll turn out to be a guy... But Alice just turns out to be a girl in guy's clothing.
Double Subverted: But she is really Carlos' sister, whom he sent to meet Bob to avoid having his cover blown.
Parodied: A virtual world is used to represent the future's version of the Internet, inhabited almost entirely by fat men in women's clothing.
Zig Zagged: Bob is starting to suspect that Alice is really a guy, but when they meet in real life, she definitely looks like a girl... And then she turns out to be a very feminine guy in girl's clothes. But then, it turns out she sent her brother/friend/boyfriend/etc. there to just mess with his head.
Alice is a girl in real life, and the thought that she might have been male had never crossed Bob's mind.
Or Carlos is a guy in real life who has a female screen name but doesn't pretend to be a girl aside from the name, and answers readily and sincerely when asked whether he is a guy or a girl in real life.
Enforced: The writer has had the exact situation occur to him, so he uses it for material.
Lampshaded: "You sure you want to meet Alice in real life, Bob? What if she turns out to be a guy?"
Invoked: Bob is depressed at the state of his romantic life, so Carlos creates a female online alter-ego, Alice, for the purpose of cheering Bob up.
Defied: Every time Bob gets involved with a girl online he asks for pictures and a way to contact her to make sure she's not a guy in real life.
Discussed: "That's the seventh time Bob has met a girl online only for her to turn out to be a guy, he really should have learned by now."
Conversed: "Let's hope this season they don't have Bob meet another GIRL. That joke's wearing pretty thin."
Deconstructed: Carlos goes online as Alice to meet guys. Unfortunately, he ends up hitting on an undercover investigator pretending to be underage who believes he is a pedophile and arrests him.
Reconstructed: The investigator is highly surprised to learn that Alice is actually Carlos.