who fucking cares? stop chatting with strangers on the internet and sending them your picture
I'm going to have to agree with this one. Generally, on the internet we're all just anonymous beings with avatars for faces. Doesn't really matter what sex you are, and you shouldn't really ever feel the need to prove it. (Unless you are on a dating site, that might be important.) Because what do you get out of it if they believe you? The satisfcation that... well, there's nothing really accomplished. Not sexism, just adhering to a meme.
I had this whole rambling spiel about women and new media and our tendency to self-label, but then I read it over and it was kind of stupid. So I guess I'll just say that maybe it's mild sexism when people say TITS OR GTFO, but I feel like the internet has enough penis jokes to make up for it. Nobody's really serious about it, and if they are, odds are you probably wouldn't be able to tell, anyway.