This doesn't mean that you can't trust anyone you meet online. It does mean that you need to exercise at least as much caution with your cyberspace acquaintances as you would with a friendly person you met at the grocery store. In fact, because it's so easy for predators to misrepresent themselves online, a little more caution is in order. And unfortunately, every cybernaut needs to remember that fellow net travelers may not be who or what they claim to be. MUDs and MOOs exist specifically for the purpose of exploring fantasy worlds and fantasy identities.
Men often represent themselves as women.
All that is just fine. Other cases are closer to the borderline of acceptable behavior. For instance, male journalists have created feminine handles to investigate sexual harassment on the net. While that isn't particularly nice, neither is sexual harassment. Netiquette permits it as long as the journalist refrains from getting deeply involved with anyone under false pretenses.