I like the idea of leaving it alone. There are folks who enjoy the ambiguity of their user name and avatar. I'm willing to bet that these same folks don't want to have to actually declare "not interested in sharing" when it comes to gender self-identification (for whatever reasons there may be for such a choice). Eventually the gender of the poster becomes clear if you are around TFP long enough and follow the boards with any sort of regularity.
If your response is gender specific then put that little addendum in the front of your post saying so. It may even make a post more interesting if a person puts in a response for each of the two major gender identifications we use. That could generate some interesting dialogue, methinks.
Personally I would like to think my manly essence permeates my posts
But that doesn't mean I want to hang it out there for all to see. As someone already said, there is enough odds and ends about the user sitting there already.