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For me (version 1.7.4.2.) this fix worked well (in the begining of the \themes\{your_template}\templates\_partials\form-fields.tpl):

Find this part

{if $field.type == 'hidden'}

  {block name='form_field_item_hidden'}
    <input type="hidden" name="{$field.name}" value="{$field.value}">
  {/block}

Add this line " or $field.name=='id_gender' " to the if parameters, to make it look like this:

{if $field.type == 'hidden' or $field.name=='id_gender'}

  {block name='form_field_item_hidden'}
    <input type="hidden" name="{$field.name}" value="{$field.value}">
  {/block}

This should influence all the filling forms globally, not only in checkout page.