How to submit form with only checkboxes using ajax with modelAttribute

I have a form with only checkboxes. I use userForm.serialize() to get form data, however, the alert printed something like "male=true&female=on&status=true" when male and status are checked. The ajax call failed with "IN ERROR", server action is not invoked. How do I submit this form using ajax so that the model attribute gets the data from JSP ? Many Thanks.

Controller:

 @RequestMapping(value = "/userSelectionSubmit",  method = { RequestMethod.POST, RequestMethod.GET })
   public ModelAndView userSubmit(@ModelAttribute UserSelections usersel)
   {
       boolean bMale = usersel.isMale();
        boolean bFemale = usersel.isFemale ();
       boolean bSingle = usersel.isSinlge();
   }

JSP (sample):

  <form:form id= "userForm" name="userForm"  method="post" 
     modelAttribute="userSelections" action="UserSelectSubmit.htm">
      <form:checkbox path="male" id="male" name="male" />male
     <form:checkbox path="female" id="female" name="female" />female
    <form:checkbox path="Single" id="Single" name="Single" />Single
    <input id='btnContinue' type="button" value="Continue"  />

    <Script>
        $(document).ready(function()
        {                                                           
            $("#btnContinue").click(function()
            {
               alert($("#userForm").serialize()); // this printed male=true&female=on&status=true   

               $.ajax({
               type: 'POST',
               url: "UserSelectSubmit.html",
               contentType: "application/json",
              data: $("#userForm").serialize()), 
               success: function(data) 
               {
                        .....
                },
                 error: function(e) 
                 {
                       alert('IN ERROR ');
                     }
             });
          })
        })