register_nav_menus, then create menus programatically

Question

There is no need to create/update custom menus programmatically, just to do what you’re trying to do.

Just rely on the wp_nav_menu() fallback, wp_page_menu(), or define your own, custom callback, e.g. to output wp_list_pages().

Calling this:

`wp_nav_menu( array( 'theme_location' => 'main-nav' ) );`

…will, if no custom menu has been assigned to the 'main-nav' theme location, output this:

`wp_page_menu()`.

If you want more fine-grained control, e.g. to control the menu depth explicitly, you could do the following:

wp_nav_menu( array(
    'theme_location' => 'main-nav',
    'depth' => 3,
    'container_class' => 'nav',
    'menu_id' => 'main-nav',
    'fallback_cb' => 'wpse87933_main_nav_cb'
) );

…and then define your callback like so:

function wpse87933_main_nav_cb() {
    ?>
    <div class="nav">
        <ul id="main-nav">
            <?php
            wp_list_pages( array(
                'depth' => 3,
                'title_li' => ''
            ) );
            ?>
        </ul>
    </div>
    <?php
}

You could even simplify things by using the has_nav_menu() conditional:

if ( has_nav_menu( 'main-nav' ) ) {
    wp_nav_menu( array(
        'theme_location' => 'main-nav',
        'depth' => 3,
        'container_class' => 'nav',
        'menu_id' => 'main-nav'
    ) );
} else {
    ?>
    <div class="nav">
        <ul id="main-nav">
            <?php
            wp_list_pages( array(
                'depth' => 3,
                'title_li' => ''
            ) );
            ?>
        </ul>
    </div>
    <?php  
}

At that point, you just need to be mindful of the CSS class differences between the 3 functions: