Disable guest checkout if virtual products are in the cart.

woocommerce-disable-guest-for-virtual.php

<?php

add_filter( 'pre_option_woocommerce_enable_guest_checkout', 'conditional_guest_checkout_based_on_product' );
function conditional_guest_checkout_based_on_product( $value ) {
  if ( WC()->cart ) {
    $cart = WC()->cart->get_cart();
    foreach ( $cart as $item ) {
      $product = $item['data'];
      if(!empty($product) && $product->is_virtual()){
        $value = "no";
        break;
      }
    }
  }

  return $value;
}