Since people are so concerned about gender, and whether or not some one is a boy or a girl, i suppose someone needs to point out that there is a third gender that should be represented as well.
Transgender is also a valid option, and I am surprised no one has mentioned this.
I personally have many friends who wouldn't classify themselves as male or female, and prefer to be identified as TG. So by limiting the options to M or F you are discriminating against the 3rd sex. Which by the way, is recognized by the government, and is more common then most people know.
I suggest that you recognize the TG community , and give people the option to choose a M, F or TG .