No need to make it clear. We don't care if you are m, f, or in between. Part of the fun is making no gender assumptions or getting them wrong. Of course some people want to be known and that's fine too, especially if you are showing up at a hot singles event. Edit I would be impressed if you were royalty though.
I am quite fashinated by the whole "Showing gender in a username" thing. I have been mistaken for a guy many times using this username. "Fun" story involving this username - My G-mail adress is the same as my username and I once had to quickly email a school assignment to a teacher. I used my personal mail because using the school one would involve hours of bullshit. The teacher was the kind of guy who had backpacked a lot and thought stories about that was an accebtable substitute for teaching. He also knew way less than he thought he did. He had mistaken badger for beaver in american slang terms. He thought this was hilarious and that I was some kind of internet escort. I lectured him on the importance and symbolism of badgers in Wisconsin. But without the "girl" at the end it wouldn't have sounded so weird.
No, he faced me about it the cafetery in front of anyone. With eyebrows. Like it was "our secret". It was mostly insinuation. One girl complimented me on what I said back to him later.
My Dad mapped out all my family's genealogy a few years back (or at least as far as he could), and I actually am a distant relative to the O'Donnell Dynasty in Ireland...What I'm saying is, we can all pretend I'm royalty if it makes you happy! I don't care either, but I try not to make assumptions about someone's gender (even though I do it subconsciously...trying to get better at that), so sometimes it is good to know how they identify :)