If you are experiencing a queer problem where by your products are automatically converting to virtual product instead of the default simple product then one of the reason might be —while creating the product, under the weight field you might be selecting the option - 'This item has no weight.' & when you do this Magento saves the products as virtual product. To save a product as simple product you must select the option as 'This item has weight.'
In case you have lots of product that are virtual and you would like to convert them to simple products, you can run this simple SQL query as below:
product-automatically-converting-simple-product-virtual-product
In case you have lots of product that are virtual and you would like to convert them to simple products, you can run this simple SQL query as below:
UPDATE catalog_product_entity SET type_id = 'simple' WHERE type_id = 'virtual'
product-automatically-converting-simple-product-virtual-product