Selected Item issue

On page load my dropdownlist contains 'F' for female and 'M' for male.  The displayed value should reflect the gender of a queried individual from a previous page.  The correct value is showing in the ddl, but for those cases where the retuning value is 'F', the listitem 'M' is not appearing as a value that I can potentially choose.  It is showing an additional 'F'.
If I reverse the order of my listitems in the following aspx then the 'M' shows twice.  Hmmm.
Here's my aspx:
<asp:DropDownList ID="cbo_gender" runat="server" Font-Names="Verdana" Font-Size="8pt" TabIndex="7"> <asp:ListItem>M</asp:ListItem> <asp:ListItem>F</asp:ListItem> </asp:DropDownList>

And here's some page load code behind: < ...