Checkbox Changed Value Doesn't Bind

The following checkbox in my FormView only records "True" to the database, even after the checkbox is unchecked. After exhausting searches for many weeks, I'm in desperate need of a resolution and would very much appreciate your expertise. From what I understand, the #Bind feature is supposed to be a 2-way binding but I don't know why it doesn't work with my checkbox controls. I know that I should use AutoPostBack="True", but when I do so, I get the error "Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control."

<asp:UpdatePanel ID="udp1" UpdateMode="Conditional" runat="server">
    <ContentTemplate>
        <asp:CheckBox ID="chkEmail" Text="Display?" TextAlign="left"
            Checked='<%#Bind("Email_Disp")%>' runat="server" /><br />
    </ContentTemplate> 
</asp:UpdatePanel> 

<asp:SqlDataSource ID="srcProfile" runat="server" ConnectionString="<%$ ConnectionStrings:webConn %>"
SelectCommand="sp_Profile_Get" SelectCommandType="StoredProcedure"
DeleteCommand="sp_Profile_Delete" DeleteCommandType="StoredProcedure"
UpdateCommand="sp_Profile_Update" UpdateCommandType="StoredProcedure">
<SelectParameters>
<asp:SessionParameter Name="UserID" SessionField="NewUserID" />
</SelectParameters>
<UpdateParameters>
<asp:Parameter Name="UserID" />
[snip]
<asp:Parameter Name="Email_Disp" Type="Boolean" />
[snip]
</UpdateParameters>
</asp:SqlDataSource>

---------
From DB table: Email_Disp    bit
---------
ALTER PROCEDURE dbo.sp_Profile_Get
@UserID uniqueidentifierAS
SELECT *
FROM [myTable] WHERE UserID = CONVERT(uniqueidentifier,@UserID)
---------
ALTER PROCEDURE dbo.sp_Profile_Update            @UserID uniqueidentifier,[snip] @Email_Disp bit,[snip]
ASUPDATE [myTable] SET [snip], Email_Disp = @Email_Disp, [snip]WHERE UserID = CONVERT(uniqueidentifier,@UserID)