Signing up to Second Life today, and the sign up form forces you to choose your gender:
This is pretty normal for registration purposes. I’ve collected quite a few snapshots of similar ‘error’ messages (will add these to the post later), from places you really would think wouldn’t care what your gender is (weather sites?!). I guess it’s good for targeted advertising to know your market, but shouldn’t it be up to the user whether they want to give out that information (without being blocked from registering)? Still, Facebook wins as the only place I remember to allow me to leave my gender blank.