PHP radio buttons and submit()

I am using On/off radio button inside my wordpress to hide post from a particular category and I’ve done well so far,

I mean when i set the button to ‘On’, posts from category whose id is 495 disappears and when i turn if off the posts comes back,

here is the problem i am facing,

I set the button to ‘Off’ so the posts from category id 495 disappears then i click on ‘older posts’ link and goes to previous pages but then when i chose ‘On’ i automatically jumped back to page number 1 :S

why is this happening ? Why am i automatically jumping back to 1st page when click ‘On’ ? :S

here is the code,

This is the header code,

<?php
session_start();
if (isset($_POST['r1'])){
    $_SESSION['r1']=$_POST['r1'];
    }
?>

these are my two radio buttons,

<li><input type="radio" name="r1" value="o" onClick="submit();" <?php echo ($_SESSION['r1'] == "o") ? 'checked="checked"' : ''; ?> />On</li>
<li><input type="radio" name="r1" value="p" onClick="submit();" <?php echo ($_SESSION['r1'] == "p") ? 'checked="checked"' : ''; ?> />Off</li>

and this is the code in index.php,

<?php 
    if ($_POST['r1']=='o') // Family Filter !!! If 'ON'
    query_posts('cat=-495'); // Remove Post from Category whose Id is 495
    else {echo "";} // When off is selected
?>

is there something wrong with the code inside index.php ??