To be fair, I think the problem in your case (and in a few other males in this site) is that you have a female avatar and this tends to make people beleive that you are female. Even if they may not think that the photo is actually of you, there is still a tendancy to associate the gender of the person in the photo with the person writing the post.

A gender symbol may help to clear up confusions but I am not sure it will necessarily stop people making assumptions from 'first impressions which are usually based on the avatar.

Its a good idea, though, and I support it being added.